Add support for conditional holidays

[ckunki]

e.g. UK and Netherlands have cases in which

• the holiday is on a fixed date
• except if the date is a specific day of week then the holiday is some days off, e.g. on a Monday

Examples

Dutch holidays contains the following code meaning: if April 27 is a Sunday then April 26, otherwise April 27:

(holiday-sexp
 '(if (zerop (calendar-day-of-week (list  4 27 year)))
      (list  4 26 year)
    (list  4 27 year))
 "Koningsdag")

Uk-holidays describe New Year bank holiday, being on January 1st except if this is SAT or SUN, then next Monday:

(defun holiday-new-year-bank-holiday ()
  (let ((m displayed-month)
        (y displayed-year))
    (calendar-increment-month m y 1)
    (when (<= m 3)
      (let ((d (calendar-day-of-week (list 1 1 y))))
        (cond ((= d 6) `(((1 3 ,y) "New Year's Day Bank Holiday")))
              ((= d 0) `(((1 2 ,y) "New Year's Day Bank Holiday"))))))))

(defun holiday-christmas-bank-holidays ()
  (let ((m displayed-month)
        (y displayed-year))
    (calendar-increment-month m y -1)
    (when (>= m 10)
      (let ((d (calendar-day-of-week (list 12 25 y))))
        (cond ((= d 5) `(((12 28 ,y) "Boxing Day Bank Holiday")))
              ((= d 6) `(((12 27 ,y) "Boxing Day Bank Holiday")
			 ((12 28 ,y) "Christmas Day Bank Holiday")))
              ((= d 0) `(((12 27 ,y) "Christmas Day Bank Holiday"))))))))

In the following table X and B are Christmas and Boxing holidays defined anyway with a fixed date.
X2 and B2 are potential additional Christmas and Boxing holidays depending on the day of week of Dec 25th.

Case	25	26	27	28	Description
	FRI	SAT	SUN	MON
1	X	B	-	B2	if DEC 25 is a FRI then 12 28 Boxing Day Bank Holiday
	SAT	SUN	MON	TUE
2	X	B	B2	X2	if DEC 25 is a SAT then 12 27 Boxing Day Bank Holiday and 12 28 Christmas Day Bank Holiday
	SUN	MON	TUE	WED
3	X	B	X2		if DEC 25 is a SUN then 12 27 Christmas Day Bank Holiday

Ideas for configuration file syntax

holiday either 4 27 or -1 if SUN Koningsdag
holiday if 4 27 is SUN then 4 26 Koningsdag
holiday except SUN 4 27 else 4 26 Koningsdag
holiday if-not SUN then 4 27 else 1 day before Koningsdag

holiday either fixed 4 27 or if SUN then fixed 4 26 Koningsdag
# leads to two holidays on Jan 1st in other cases
holiday either fixed JAN 1 or if SAT,SUN then float 1 MON after JAN 1 New Year's Day Bank Holiday
# leads to two Boxing holidays on Dec 26 in other cases
holiday either fixed DEC 26 or if SAT,SUN then float 1 MON after DEC 26 Boxing Day Bank Holiday
# leads to two Christmas holidays on Dec 25 in other cases
holiday either fixed DEC 25 or if SAT,SUN then float 1 TUE after DEC 25 Christmas Day Bank Holiday

[ckunki]

Maybe we should make a difference between conditional holidays and holidays with alternative dates?

• conditional holidays may be skipped in particular years without replacement, example
  holiday if DEC 26 is SAT,SUN then float 1 MON after DEC 26 Boxing Day Bank Holiday
• holidays with alternative dates occur every year, but date may be different, example
  holiday either 4 27 or if SUN then fixed 4 26 Koningsdag