easter-date.lisp

(defun easter-date-for-year (year &optional &key (use-date-name t))
  (let ((golden-year (+ 1 (mod year 19)))
	(century (+ (/ year 100) 1)))
    (let ((skipped-leap-years (- (/ (* 3 century) 4) 12))
	  (correction-factor (- (/ (+ (* 8 century) 5) 25) 5)))
      (let ((d (- (/ (* 5 year) 4) skipped-leap-years 10))
	    (epac (mod (- (+ (* 11 golden-year) 20 correction-factor) skipped-leap-years) 30)))
 
	(if (or (and (= epac 25) (> golden-year 11)) (= epac 24))
	    (incf epac))
 
	(let ((n (- 44 epac)) (month nil) (day-of-month nil))
	  (if (< n 21)
	      (setf n (+ 30 n)))
 
	  (setf n (- (+ n 7) (mod (+ d n) 7)))
 
	  (cond ((> n 31)
		 (setf month
		       (if use-date-name
			   'APRIL
			   4))
		 (setf day-of-month (- n 31)))
		(t
		 (setf month
		       (if use-date-name
			   'MARCH
			   3))
		 (setf day-of-month n)))
 
	  (let ((formatted-date (if use-date-name
				    "~a. ~a ~a"
				    "~a.~a.~a")))
	    (format nil 
		    (concatenate 'string "Easter date: " formatted-date) 
		    (round day-of-month) month year)))))))