+File: xemacs.info, Node: Holiday Customizing, Next: Date Display Format, Prev: Calendar Customizing, Up: Calendar Customization
+
+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. MONTH and DAY are
+ numbers, STRING is the name of the holiday.
+
+`(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. STRING is the name of the holiday.
+
+`(holiday-hebrew MONTH DAY STRING)'
+ A fixed date on the Hebrew calendar. MONTH and DAY are numbers,
+ STRING is the name of the holiday.
+
+`(holiday-islamic MONTH DAY STRING)'
+ A fixed date on the Islamic calendar. MONTH and DAY are numbers,
+ STRING is the name of the holiday.
+
+`(holiday-julian MONTH DAY STRING)'
+ A fixed date on the Julian calendar. MONTH and DAY are numbers,
+ STRING is the name of the holiday.
+
+`(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)'. STRING is the name of the holiday.
+
+`(if CONDITION HOLIDAY-FORM &optional 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 by adding the following line to
+your init file:
+
+ (setq other-holidays '((holiday-fixed 7 14 "Bastille Day")))
+
+ *Note Init File::.
+
+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") ... )
+
+\1f
+File: xemacs.info, Node: Date Display Format, Next: Time Display Format, Prev: Holiday Customizing, Up: Calendar Customization
+
+Date Display Format
+...................
+
+ You can customize the manner of displaying dates in the diary, in
+mode lines, and in messages by setting `calendar-date-display-form'.
+This variable holds a list of expressions that can involve the variables
+`month', `day', and `year', which are all numbers in string form, and
+`monthname' and `dayname', which are both alphabetic strings. In the
+American style, the default value of this list is as follows:
+
+ ((if dayname (concat dayname ", ")) monthname " " day ", " year)
+
+while in the European style this value is the default:
+
+ ((if dayname (concat dayname ", ")) day " " monthname " " year)
+
+ + The ISO standard date representation is this:
+
+ (year "-" month "-" day)
+
+This specifies a typical American format:
+
+ (month "/" day "/" (substring year -2))
+
+\1f
+File: xemacs.info, Node: Time Display Format, Next: Daylight Savings, Prev: Date Display Format, Up: Calendar Customization
+
+Time Display Format
+...................
+
+ The calendar and diary by default display times of day in the
+conventional American style with the hours from 1 through 12, minutes,
+and either `am' or `pm'. If you prefer the European style, also known
+in the US as military, in which the hours go from 00 to 23, you can
+alter the variable `calendar-time-display-form'. This variable is a
+list of expressions that can involve the variables `12-hours',
+`24-hours', and `minutes', which are all numbers in string form, and
+`am-pm' and `time-zone', which are both alphabetic strings. The
+default value of `calendar-time-display-form' is as follows:
+
+ (12-hours ":" minutes am-pm
+ (if time-zone " (") time-zone (if time-zone ")"))
+
+Here is a value that provides European style times:
+
+ (24-hours ":" minutes
+ (if time-zone " (") time-zone (if time-zone ")"))
+
+gives military-style times like `21:07 (UT)' if time zone names are
+defined, and times like `21:07' if they are not.
+
+\1f
+File: xemacs.info, Node: Daylight Savings, Next: Diary Customizing, Prev: Time Display Format, Up: Calendar Customization
+
+Daylight Savings Time
+.....................
+
+ Emacs understands the difference between standard time and daylight
+savings time--the times given for sunrise, sunset, solstices,
+equinoxes, and the phases of the moon take that into account. The rules
+for daylight savings time vary from place to place and have also varied
+historically from year to year. To do the job properly, Emacs needs to
+know which rules to use.
+
+ Some operating systems keep track of the rules that apply to the
+place where you are; on these systems, Emacs gets the information it
+needs from the system automatically. If some or all of this
+information is missing, Emacs fills in the gaps with the rules
+currently used in Cambridge, Massachusetts. If the resulting rules are
+not what you want, you can tell Emacs the rules to use by setting
+certain variables.
+
+ If the default choice of rules is not appropriate for your location,
+you can tell Emacs the rules to use by setting the variables
+`calendar-daylight-savings-starts' and
+`calendar-daylight-savings-ends'. Their values should be Lisp
+expressions that refer to the variable `year', and evaluate to the
+Gregorian date on which daylight savings time starts or (respectively)
+ends, in the form of a list `(MONTH DAY YEAR)'. The values should be
+`nil' if your area does not use daylight savings time.
+
+ Emacs uses these expressions to determine the starting date of
+daylight savings time for the holiday list and for correcting times of
+day in the solar and lunar calculations.
+
+ The values for Cambridge, Massachusetts are as follows:
+
+ (calendar-nth-named-day 1 0 4 year)
+ (calendar-nth-named-day -1 0 10 year)
+
+That is, the first 0th day (Sunday) of the fourth month (April) in the
+year specified by `year', and the last Sunday of the tenth month
+(October) of that year. If daylight savings time were changed to start
+on October 1, you would set `calendar-daylight-savings-starts' to this:
+
+ (list 10 1 year)
+
+ For a more complex example, suppose daylight savings time begins on
+the first of Nisan on the Hebrew calendar. You should set
+`calendar-daylight-savings-starts' to this value:
+
+ (calendar-gregorian-from-absolute
+ (calendar-absolute-from-hebrew
+ (list 1 1 (+ year 3760))))
+
+because Nisan is the first month in the Hebrew calendar and the Hebrew
+year differs from the Gregorian year by 3760 at Nisan.
+
+ If there is no daylight savings time at your location, or if you want
+all times in standard time, set `calendar-daylight-savings-starts' and
+`calendar-daylight-savings-ends' to `nil'.
+
+ The variable `calendar-daylight-time-offset' specifies the
+difference between daylight savings time and standard time, measured in
+minutes. The value for Cambridge, Massachusetts is 60.
+
+ The two variables `calendar-daylight-savings-starts-time' and
+`calendar-daylight-savings-ends-time' specify the number of minutes
+after midnight local time when the transition to and from daylight
+savings time should occur. For Cambridge, Massachusetts both variables'
+values are 120.
+
+\1f