Although options offically expire on the Saturday immediately following the third Friday of the expiration month, for most mortals, that means the option expires the third Friday, since your friendly neighborhood broker or internet trading company won't talk to you on Saturday. The broker-broker settlements are done effective Saturday. While most trades consider only weekdays as business days, the Saturday following the third Friday is a business day for expiring options. Stock index futures, stock index options and options on individual stocks all expire on this day, and because of this, trading volume is usually especially high on the stock exchanges that day. In 1987, the expiration of key index contracts was changed from the close of trading on that day to the open of trading on that day, which helped reduce the volatility of the markets somewhat by giving specialists more time to match orders.
Volume is quite simply the number of contracts traded on a given day.
The open interest is slightly more complicated. The open interest figure for a given option is the number of contracts outstanding at a given time. When trader A closes out the position by selling the option, the open interest either remain the same or go down. If A sells to someone who did not have a position before, or was already long, the open interest does not change. If A sells to someone who had a short position, the open interest decreases by one.