The NYSE and NASDAQ average about 251 trading days a year.
If Juneteenth, Independence Day, Christmas Day and New Year's Day fall on a weekend, the public holiday is observed on the preceding Friday (for a Sarturday holiday) or the following Monday (For a Sunday holiday) instead, meaning that every holiday results in one fewer trading day.
[1] Up to three trading days (July 3, the day after Thanksgiving, and Christmas Eve) are shortened, i.e. the exchanges are open from 9:30AM–1:00PM, depending on where they fall in the calendar year (if July 3 or Christmas Eve fall on a weekend, the shortened day is simply skipped).
[2] Each stock exchange has opening hours that are based on specific time zones.
During the part of the year when North America is on standard time, it would be 17:30–24:00 in Moscow, and in Shanghai it would be 22:30–5:00.