An Option belongs to a particular series if it belongs to a same class (call or put) same strike price and same expiry month. If you are long in a particular option, your position can be
squared off, if you sell options in the same series.
Expiration Period (Or length of the contract)
Normally expiration periods are calendar months. Contract are available for current month and next two months. For example, if today is 6th June, then options are June, July and August.
The last date for trading options in a particular month is known as expiry date. Last Thursday of every month is expiry date for that month’s options. ( Previous day if Thursday is a Holiday)
