The short answer is that you can never tell when you will be assigned. Once you sell an option (put or call), you have the potential for being assigned to fulfill your obligation to receive (and pay for) or deliver (and get paid for) shares of stock on any business day. In some circumstances, you may be assigned on a short option position while the underlying shares are halted for trading, or perhaps while they are the subjects of a buyout or takeover. To ensure fairness in the distribution of equity and index option assignments, The Options Clearing Corporation utilizes a random procedure to assign exercise notices to the accounts maintained with OCC by each clearing member. The assigned firm must then use an exchange approved method (usually a random process or the "first-in, first-out" method) to allocate those notices to accounts which are short the options.
 
Having said that, there are some generalizations which might help you understand when you might be more likely to be assigned on a short-option position.
 
Only about 17% of options end up being exercised; the percentage hasn't varied much over the years. That does not mean that you can only be assigned on 17% of your short option, however. It means that, in general, option exercises are not that common.
 
The majority of option exercises (and the corresponding assignments) take place as the option gets closer to expiration. Without getting into the math too much, it usually doesn't make sense to exercise an option, which has any time premium over intrinsic value. For most options, that doesn't occur until close to expiration.
 
In general terms, a put which goes in-the-money is more likely to be exercised than a call which goes in-the-money. Why? Think about the result of an exercise. An investor who exercises a put uses it to sell shares and receive cash. A person exercising a call option uses it to buy shares and must pay cash. People are more likely to exercise options if it means they can receive cash sooner. The opposite is true for calls, where exercise means you have to pay cash sooner.
 
The bottom line is that you really don't have any sure-fire way to predict when you will be assigned on a short option position; it can happen any day the stock market is open for trading.
 

 
The option holder has the right to exercise his or her options position prior to expiration regardless of whether the options are in- or out-of-the-money. You can be assigned if an investor or market professional holding calls of the same series as your short position submits an exercise notice to his or her brokerage firms, which in turn, submitted an exercise notice to the Options Clearing Corporation (OCC) (or if the brokerage firm is not an OCC clearing member, then it would submit the notice to a firm that is an OCC clearing member, and that member would then submit the notice to OCC). OCC randomly assigns exercise notices to clearing members in whose accounts have short positions of the same series. The clearing member then assigns the exercise to one of its short positions using a fair assignment method, though not necessarily random. You should ask your brokerage firm how it assigns exercise notices to its customers.
 

 
No, it is not possible. The assignments are determined based on net positions after the close of the market each day. Therefore, if you bought back your short call, you no longer have a short position at the end of the day, and therefore no possibility of being assigned.
 

 
The exercise of an option prior to expiration is solely at the discretion of the buyer. The option buyer can also decide how many contracts in a multi-contract position to exercise at a given time. Once an exercise notice is tendered, the Options Clearing Corporation randomly selects for assignment a member brokerage firm carrying a short position in that series. The brokerage firm may, in turn, assign the notice randomly, or on a "first-in, first-out" basis. Regardless of what method is applied by the brokerage firm, equity options writers are subject to the risk each day that some or all of their short options may be assigned.
 

 
OCC's clearing members can submit exercise notices, on a daily basis, up to 7:00 p.m. Central time, and in general each clearing member will establish its own (earlier) deadline for its customers. (There is a different set of procedures for expiring options.) As part of its nightly processing, OCC randomly assigns its clearing members based on the day's exercises. This processing is completed at approximately 1:00 a.m. Central time. OCC transmits the assignments to its clearing members and as part of the member's nightly processing, they allocate the assignments to their customers on either a random basis or a first in - first out basis.
 
We understand the exchanges' rules are that customers should be notified of assignments in a timely manner. It is best that you and your broker determine what constitutes "timely manner" beforehand: does "timely manner" mean a call the morning of the assignment? Is there an update to your account that you can view online? Will you receive a letter in the mail?
 
For expirations, OCC process all exercise/assignments on Saturday. OCC's processing is completed and transmitted to its clearing members late Saturday night. Clearing members will process expiration exercises and assignments on Sunday and then notify their customers the next business day. Once again, you can probably get an approximation from your broker.
 

 
While each firm may have their own thresholds, the option clearing corporation's auto exercise threshold is 1 cent ($0.01). Customers and brokers should check with their firm's operations department to determine their company's policies regarding exercise thresholds. An option holder has the right to exercise their option regardless of the price of the underlying security. It might be a good practice for all option holders to express their exercise - or non-exercise - instructions to their broker. Is there a magic number that ensures that option writers will not be assigned? The answer would be 'NO'. Although unlikely, an investor may choose to exercise a slightly out of the money option or choose not to exercise an option that is in the money by greater than 1 cent (.01).

Some investors use the saying, "when in doubt, close them out". This means that if they buy back any short contracts they are no longer at risk of being assigned.
 

 
The most obvious and straight forward action would be to close out the position by buying the call back. While this may not be attractive and may result in a loss, or less than hoped for gain, it will assure that your stock will not be called away. Some alternatives to being assigned are to "roll out and up". To roll out and up involves buying back the current option and selling a higher strike in a further out month. This may allow an investor to gain some additional time premium and added stock appreciation. You will want to consult your broker for any advice on this strategy.
 

 
At the end of the day, the Options Clearing Corporation accumulates all option exercises. Prior to processing exercises, OCC's clearing system accumulates all post-trade transactions and matched-trades together and applies them to clearing members' positions in the following sequence:
1) All opening buys
2) All opening sells
3) All closing buys
4) Exercises
5) All closing sells
6) Assignments
 
Because closing buys are processed before exercises, there is no possibility of assignment on positions that were closed via closing buy orders during that day's trading hours.
 

 
No. There are several reasons why this is untrue. First, the buy side of your opening sale could have been a closing purchase by someone who was already short the option. Second, as assignments are handed out randomly, anyone who is short that particular option is at risk of being assigned when an option holder exercises. Third, assuming the other side of your trade was an opening purchase, they may sell to close anytime but since you are still short, you are at risk of being assigned. As long as you keep a short option position open, you are at risk of being assigned. Generally speaking, assignment risk increases as the option becomes deeper in the money and as expiration approaches (the option trades with less time premium). Assignment risk also increases just before the ex-dividend date for short calls and just after the ex-dividend date for short puts.
 
At expiration all equity options that are in-the-money by $0.01 or more are exercised unless the option holder instructs their broker not to exercise.
 

"VIX was designed to be a consistent, 30-day benchmark of expected market volatility, as measured by SPX option prices. Of course, there is only one day in the life of any option that is exactly 30 days to expiration, so in order to arrive at the 30-day standard, VIX is calculated as a weighted average of options expiring on two different dates.

One day each month, on the Wednesday that is thirty days prior to the third Friday of the following calendar month, the SPX options expiring in exactly 30 days account for all of the weight in the VIX calculation. VIX options settle on these Wednesdays in order to facilitate the special opening procedures that establish opening prices for those SPX options used to calculate the exercise settlement value for VIX options."

(Source CBOE FAQs: http://www.cboe.com/micro/vix/vixoptionsfaq.aspx#4)

Here is a list of all VIX options expiration dates through December 2010.
Please note: trading will cease on the business day before the expiration date.

2009 Expiration Dates
Wednesday, January 21, 2009
Wednesday, February 18, 2009
Wednesday, March 18, 2009
Wednesday, April 15, 2009
Wednesday, May 20, 2009
Wednesday, June 17, 2009
Wednesday, July 22, 2009
Wednesday, August 19, 2009
Wednesday, September 16, 2009
Wednesday, October 21, 2009
Wednesday, November 18, 2009
Wednesday, December 16, 2009

2010 Expiration Dates
Wednesday, January 20, 2010
Wednesday, February 17, 2010
Wednesday, March 17, 2010
Wednesday, April 21, 2010
Wednesday, May 19, 2010
Wednesday, June 16, 2010
Wednesday, July 21, 2010
Wednesday, August 18, 2010
Wednesday, September 15, 2010
Wednesday, October 20, 2010
Wednesday, November 17, 2010
Wednesday, December 22, 2010

Options normally trade in increments of 5 or 10 cents depending on the value of the underlying.

However...

The SEC has requested that the options exchanges commence a pilot to trade options in one cent increments. This program will continue to expand through out the year.

The pilot allows for trading increments of one cent for bids/offers up to $3 and 5 cents for bids/offers above $3, for each of the symbols in the pilot except for QQQQ. The symbol QQQQ will trade in one-cent increments for all prices.

Each exchange either has filed or will file rules that describe the trading increments for the pilot stocks, changes that result from one-cent increments, various reports to measure the impact of the pilot, and plans for quote mitigation.

You can exercise your options by calling us (877-495-5464) on any day during regular business hours up to 4:30 pm ET. The resulting position will be reflected in your account on the same day of your exercise.
 
On regular exercise day (the Saturday following the third Thursday of each month), equity options and index options with a value of $.01 or more will be subject to automatic exercise and/or assignment. On Saturday your in-the-money options will be removed from your account and on Sunday the resulting equity or index position will be reflected in your account.
 
Please note that if you are holding in-the-money options and you do not have enough equity or you do not own the underlying, you will be responsible to cover the resulting long or short position on Monday morning. If your option is in the money but you do not want to be subjected to automatic exercise, please contact us by 4:30 pm ET on the Friday of expiration. If you do not have enough equity in your account to cover an automatic exercise, TradeKing may at its discretion let the option expire with no exercise even if, theoretically, it should have been subjected to automatic exercise.

Non standard options are options that are subject to special settlement due to an underlying company reorganization, stock split, merger, or special dividend. Often non-standard options represent more than the usual 100 shares lot, other times they may represent the exercise of lots of different securities.
 
You can access a daily report of special settlements at the Options Clearing Corporation's website.

These values may be found on the CBOE website http://cboe.com/data/Settlement.aspx. Please note that the amounts may not be known until late in the afternoon, on the last trading day of expiration week.