The Emacs calendar knows about all major and many minor holidays.
To see if any holidays fall on a given date, position point on that date in the calendar window and use the h command. The holidays are usually listed in the echo area, but if there are too many to fit in one line, then they are displayed in a separate window.
To find the distribution of holidays for a wider period, you can use the x command to mark all the days in the calendar display that have holidays. The mark appears as a `*' next to the date in the calendar. This command applies to the dates currently visible and new dates that become visible by scrolling new months onto the display. To erase these marks from the calendar, use u, which also erases any diary marks (see section The Diary); neither holidays nor diary entries will be marked on new dates that are scrolled into view.
To get even more detailed information, use the a command, which displays a separate buffer containing a list of all holidays in the current three-month range.
You can display the list of holidays for the current month and the preceding and succeeding months even if you don't have a calendar window. Use the command M-x holidays.
The holidays known to Emacs include American holidays, as well as major Christian, Jewish, and Islamic holidays. The dates used by Emacs for holidays are based on current practice, not historical fact. Historically, for instance, the start of daylight savings time and even its existence have varied from year to year. However present American law mandates that daylight savings time begins on the first Sunday in April; this is the definition that Emacs uses, even though it will be wrong for some prior years.
The religious holidays known to Emacs are usually limited to those
commonly found in secular calendars. For a more extensive collection of
religious holidays, you can set any (or all) of the variables
t in your `.emacs'
You can easily customize the list of holidays to your own needs; to find out how, use C-h v calendar-holidays RET after entering the calendar.