Получить день недели. Отсчёт от воскресенья: 0..6.

(defun day-of-week (day month year)
  "Returns the day of the week as an integer.
   Sunday is 0. Works for years after 1752."
  (let ((offset '(0 3 2 5 0 3 5 1 4 6 2 4)))
    (when (< month 3)
      (decf year 1))
    (mod
     (truncate (+ year
                  (/ year 4)
                  (/ (- year)
                     100)
                  (/ year 400)
                  (nth (1- month) offset)
                  day
                  -1))
     7)))
; DAY-OF-WEEK

(day-of-week 23 12 1965)
; 3

(day-of-week 1 1 1900)
; 0

(day-of-week 31 12 1899)
; 6