Trading day:
In business, the trading day is the time span that a particular stock exchange is open. For example, the New York Stock Exchange is, as of 2008, opens from 9:30 AM Eastern time to 4:00 PM Eastern time.
Trading days never take place on weekends.
When a trading day ends, all share trading ends and is frozen in time until the next trading day begins.
There are several other special circumstances which would lead to a shortened trading day, or no trading day at all, such as on holidays.