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(elisp)Holiday Customizing

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Customizing the Holidays
========================

   Emacs knows about holidays defined by entries on one of several
lists.  You can customize these lists of holidays to your own needs,
adding or deleting holidays.  The lists of holidays that Emacs uses are
for general holidays (`general-holidays'), local holidays
(`local-holidays'), Christian holidays (`christian-holidays'), Hebrew
(Jewish) holidays (`hebrew-holidays'), Islamic (Moslem) holidays
(`islamic-holidays'), and other holidays (`other-holidays').

   The general holidays are, by default, holidays common throughout the
United States.  To eliminate these holidays, set `general-holidays' to
`nil'.

   There are no default local holidays (but sites may supply some).  You
can set the variable `local-holidays' to any list of holidays, as
described below.

   By default, Emacs does not include all the holidays of the religions
that it knows, only those commonly found in secular calendars.  For a
more extensive collection of religious holidays, you can set any (or
all) of the variables `all-christian-calendar-holidays',
`all-hebrew-calendar-holidays', or `all-islamic-calendar-holidays' to
`t'.  If you want to eliminate the religious holidays, set any or all
of the corresponding variables `christian-holidays', `hebrew-holidays',
and `islamic-holidays' to `nil'.

   You can set the variable `other-holidays' to any list of holidays.
This list, normally empty, is intended for individual use.

   Each of the lists (`general-holidays', `local-holidays',
`christian-holidays', `hebrew-holidays', `islamic-holidays', and
`other-holidays') is a list of "holiday forms", each holiday form
describing a holiday (or sometimes a list of holidays).

   Here is a table of the possible kinds of holiday form.  Day numbers
and month numbers count starting from 1, but "dayname" numbers count
Sunday as 0.  The element STRING is always the name of the holiday, as
a string.

`(holiday-fixed MONTH DAY STRING)'
     A fixed date on the Gregorian calendar.

`(holiday-float MONTH DAYNAME K STRING)'
     The Kth DAYNAME in MONTH on the Gregorian calendar (DAYNAME=0 for
     Sunday, and so on); negative K means count back from the end of
     the month.

`(holiday-hebrew MONTH DAY STRING)'
     A fixed date on the Hebrew calendar.

`(holiday-islamic MONTH DAY STRING)'
     A fixed date on the Islamic calendar.

`(holiday-julian MONTH DAY STRING)'
     A fixed date on the Julian calendar.

`(holiday-sexp SEXP STRING)'
     A date calculated by the Lisp expression SEXP.  The expression
     should use the variable `year' to compute and return the date of a
     holiday, or `nil' if the holiday doesn't happen this year.  The
     value of SEXP must represent the date as a list of the form
     `(MONTH DAY YEAR)'.

`(if CONDITION HOLIDAY-FORM)'
     A holiday that happens only if CONDITION is true.

`(FUNCTION [ARGS])'
     A list of dates calculated by the function FUNCTION, called with
     arguments ARGS.

   For example, suppose you want to add Bastille Day, celebrated in
France on July 14.  You can do this as follows:

     (setq other-holidays '((holiday-fixed 7 14 "Bastille Day")))

The holiday form `(holiday-fixed 7 14 "Bastille Day")' specifies the
fourteenth day of the seventh month (July).

   Many holidays occur on a specific day of the week, at a specific time
of month.  Here is a holiday form describing Hurricane Supplication Day,
celebrated in the Virgin Islands on the fourth Monday in August:

     (holiday-float 8 1 4 "Hurricane Supplication Day")

Here the 8 specifies August, the 1 specifies Monday (Sunday is 0,
Tuesday is 2, and so on), and the 4 specifies the fourth occurrence in
the month (1 specifies the first occurrence, 2 the second occurrence,
-1 the last occurrence, -2 the second-to-last occurrence, and so on).

   You can specify holidays that occur on fixed days of the Hebrew,
Islamic, and Julian calendars too.  For example,

     (setq other-holidays
           '((holiday-hebrew 10 2 "Last day of Hanukkah")
             (holiday-islamic 3 12 "Mohammed's Birthday")
             (holiday-julian 4 2 "Jefferson's Birthday")))

adds the last day of Hanukkah (since the Hebrew months are numbered with
1 starting from Nisan), the Islamic feast celebrating Mohammed's
birthday (since the Islamic months are numbered from 1 starting with
Muharram), and Thomas Jefferson's birthday, which is 2 April 1743 on the
Julian calendar.

   To include a holiday conditionally, use either Emacs Lisp's `if' or
the `holiday-sexp' form.  For example, American presidential elections
occur on the first Tuesday after the first Monday in November of years
divisible by 4:

     (holiday-sexp (if (= 0 (% year 4))
                        (calendar-gregorian-from-absolute
                         (1+ (calendar-dayname-on-or-before
                               1 (+ 6 (calendar-absolute-from-gregorian
                                       (list 11 1 year))))))
                   "US Presidential Election"))

or

     (if (= 0 (% displayed-year 4))
         (fixed 11
                (extract-calendar-day
                  (calendar-gregorian-from-absolute
                    (1+ (calendar-dayname-on-or-before
                          1 (+ 6 (calendar-absolute-from-gregorian
                                   (list 11 1 displayed-year)))))))
                "US Presidential Election"))

   Some holidays just don't fit into any of these forms because special
calculations are involved in their determination.  In such cases you
must write a Lisp function to do the calculation.  To include eclipses,
for example, add `(eclipses)' to `other-holidays' and write an Emacs
Lisp function `eclipses' that returns a (possibly empty) list of the
relevant Gregorian dates among the range visible in the calendar
window, with descriptive strings, like this:

     (((6 27 1991) "Lunar Eclipse") ((7 11 1991) "Solar Eclipse") ... )