You can never tell when you will be assigned. Once you sell an American-style option (put or call), you have the potential for assignment to fulfill your obligation to receive (and pay for) or deliver (and are 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, OCC utilizes a random procedure to assign exercise notices to clearing member accounts maintained with OCC. The assigned firm must then use an exchange-approved method (usually a random process or the first-in, first-out method) to allocate notices to its accounts that are short the options.
Some generalizations might help you understand likelihood of assignment on a short-option position:
- Option holders only exercise about 7% of options. The percentage hasn't varied much over the years. That does not mean that you can only be assigned on 7% of your short option. It means that, in general, option exercises are not that common.
- The majority of option exercises (and the corresponding assignments) occurs as the option gets closer to expiration. 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, an investor is more likely to exercise a put that goes in-the-money than a call that 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. Option holders 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.