Emacs Lisp function to check whether the current date is Friday the 13th with full moon

friday-the-13th-full-moon.el

(require 'cl)

(defun friday-the-13th-full-moon-p ()
  "Return true if the current day is Friday the 13th with a full moon."
  (let ((value)
        (current-day (nth 3 (decode-time)))
        (current-month (nth 4 (decode-time)))
        (current-year (nth 5 (decode-time)))
        (full-moon 2))
    (dolist (elt (lunar-phase-list current-month current-year) value)
      (let ((year (caddar elt))
            (day (cadar elt))
            (month (caar elt))
            (lunar-phase (car (last elt))))
        (when (and (eq day current-day)
                   (eq month current-month)
                   (eq year current-year)
                   (eq lunar-phase full-moon))
          (setq value 't))))))

(provide 'friday-the-13th-full-moon-p)