Trading day
In business, the trading day or regular trading hours is the time span that a stock exchange is open, as opposed to electronic or extended trading hours. For example, the New York Stock Exchange is, as of the year 2020, open from 9:30 AM Eastern Time to 4:00 PM Eastern Time. Trading days are usually Monday through Friday. When a trading day ends, all trading ends and is frozen in time until the next trading day begins. There are several special circumstances which would lead to a shortened trading day, or no trading day at all, such as on holidays or on days when a state funeral of a head of state is scheduled to take place.
NYSE and NASDAQ
The NYSE and NASDAQ average about 251 trading days a year. This is from 365.2425 * 5/7 - 10 = 250.8875 ≈ 251.The holidays where the stock exchange is closed are New Year's Day, Martin Luther King Jr. Day, Presidents' Day, Good Friday, Memorial Day, Juneteenth, Independence Day, Labor Day, Thanksgiving Day, and Christmas Day; there are also some holidays where trading is permitted, including Columbus Day, Veterans Day, and New Year's Eve. If Juneteenth, Independence Day, Christmas Day and New Year's Day fall on a weekend, the public holiday is observed on the preceding Friday or the following Monday instead, meaning that every holiday results in one fewer trading day.
Up to three trading days are shortened, i.e. the exchanges are open from 9:30AM–1:00PM, depending on where they fall in the calendar year.
Juneteenth was declared a national holiday on June 17, 2021. It was first observed as a national holiday and a market holiday on June 20, 2022.