+(defconstant* +zellers-adj+ #(0 3 2 5 0 3 5 1 4 6 2 4))
+
+(defun day-of-week (year month day)
+ "Day of week calculation using Zeller's Congruence.
+Input: The year y, month m (1 ≤ m ≤ 12) and day d (1 ≤ d ≤ 31).
+Output: n - the day of the week (Sunday = 0, Saturday = 6)."
+
+ (when (< month 3)
+ (decf year))
+ (mod
+ (+ year (floor year 4) (- (floor year 100)) (floor year 400)
+ (aref +zellers-adj+ (1- month)) day)
+ 7))
+
+;;;; Daylight Saving Time calculations