holidays.el

[Håkon Malmedal]

I was thinking that if the function holiday-easter-etc looked
something like this:

(defun holiday-easter-etc (n string)
  "Holiday on Nth day after Easter called STRING."
  (let* ((century (1+ (/ displayed-year 100)))
         (shifted-epact        ;; Age of moon for April 5...
          (% (+ 14 (* 11 (% displayed-year 19));;     ...by Nicaean rule
                (-           ;; ...corrected for the Gregorian century rule
                 (/ (* 3 century) 4))
                (/    ;; ...corrected for Metonic cycle inaccuracy.
                 (+ 5 (* 8 century)) 25)
                (* 30 century));;              Keeps value positive.
             30))
         (adjusted-epact       ;;  Adjust for 29.5 day month.
          (if (or (= shifted-epact 0)
                  (and (= shifted-epact 1) (< 10 (% displayed-year 19))))
              (1+ shifted-epact)
            shifted-epact))
         (paschal-moon       ;; Day after the full moon on or after March 21.
          (- (calendar-absolute-from-gregorian (list 4 19 displayed-year))
             adjusted-epact))
         (abs-easter (calendar-dayname-on-or-before 0 (+ paschal-moon 7)))
         (mandatory
          (list
           (list (calendar-gregorian-from-absolute (+ abs-easter n))
                 string)))
         (output-list
          (filter-visible-calendar-holidays mandatory)))
    output-list))

and the function holiday-advent looked something like this:

(defun holiday-advent (n string)
  "Holiday on Nth day after Advent called STRING."
  (let ((year displayed-year)
        (month displayed-month))
    (increment-calendar-month month year -1)
    (let ((advent (calendar-gregorian-from-absolute
                   (+ n
                    (calendar-dayname-on-or-before 0
                     (calendar-absolute-from-gregorian
                      (list 12 3 year)))))))
      (if (calendar-date-is-visible-p advent)
          (list (list advent string))))))

then the variable christian-holidays (in calendar.el) could look like
this:

;;;###autoload
(put 'christian-holidays 'risky-local-variable t)
;;;###autoload
(defcustom christian-holidays
  '((if all-christian-calendar-holidays
        (holiday-fixed 1 6 "Epiphany"))
    (holiday-easter-etc 0 "Easter Sunday")
    (holiday-easter-etc -2 "Good Friday")
    (holiday-easter-etc -46 "Ash Wednesday")
    (if all-christian-calendar-holidays
        (holiday-easter-etc -63 "Septuagesima Sunday"))
    (if all-christian-calendar-holidays
        (holiday-easter-etc -56 "Sexagesima Sunday"))
    (if all-christian-calendar-holidays
        (holiday-easter-etc -49 "Shrove Sunday"))
    (if all-christian-calendar-holidays
        (holiday-easter-etc -48 "Shrove Monday"))
    (if all-christian-calendar-holidays
        (holiday-easter-etc -47 "Shrove Tuesday"))
    (if all-christian-calendar-holidays
        (holiday-easter-etc -14 "Passion Sunday"))
    (if all-christian-calendar-holidays
        (holiday-easter-etc -7 "Palm Sunday"))
    (if all-christian-calendar-holidays
        (holiday-easter-etc -3 "Maundy Thursday"))
    (if all-christian-calendar-holidays
        (holiday-easter-etc 35 "Rogation Sunday"))
    (if all-christian-calendar-holidays
        (holiday-easter-etc 39 "Ascension Day"))
    (if all-christian-calendar-holidays
        (holiday-easter-etc 49 "Pentecost (Whitsunday)"))
    (if all-christian-calendar-holidays
        (holiday-easter-etc 50 "Whitmonday"))
    (if all-christian-calendar-holidays
        (holiday-easter-etc 56 "Trinity Sunday"))
    (if all-christian-calendar-holidays
        (holiday-easter-etc 60 "Corpus Christi"))
    (if all-christian-calendar-holidays
        (holiday-greek-orthodox-easter))
    (if all-christian-calendar-holidays
        (holiday-fixed 8 15 "Assumption"))
    (if all-christian-calendar-holidays
        (holiday-advent 0 "Advent"))
    (holiday-fixed 12 25 "Christmas")
    (if all-christian-calendar-holidays
        (holiday-julian 12 25 "Eastern Orthodox Christmas")))
  "*Christian holidays.
See the documentation for `calendar-holidays' for details."
  :type 'sexp
  :group 'holidays)

and be much easier to customize. Is this a good/bad idea?