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 2015, open from 9:30 AM Eastern Time to 4:00 PM Eastern Time. Trading days are usually Monday to Friday. 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.
The NYSE has about 252 trading days in a year. This is from 365.25(days on average per year) * 5/7(proportion work days per week) = 260.89 - 9(holidays) = 251.89 ~ 252. The holidays are New Year's Day, Martin Luther King Jr. Day, Washington's Birthday, Good Friday, Memorial Day, Independence Day, Labor Day, Thanksgiving Day, Christmas Day.[1] Three of the trading days (the days before Independence Day, Thanksgiving, and Christmas Day) are shortened i.e. open from 9:30AM-1:00PM.