(require 'calendar)

(defvar my-dow-date-regexp
  "\\(?:\\([0-9]+\\)[年/-]\\)?\\([01]?[0-9]\\)[月/-]\\([0123]?[0-9]\\)[日]?[ 　]*[(（]\\([^)）]+\\)[)）]")
(defvar my-dow-index-year         1)
(defvar my-dow-index-month        2)
(defvar my-dow-index-day-of-month 3)
(defvar my-dow-index-name         4)
(defvar my-dow-day-regexp-alist
  (let ((alist '()))
    (dolist (spec
             '((0 "sun" "日")
               (1 "mon" "月")
               (2 "tue" "火")
               (3 "wed" "水")
               (4 "thu" "木")
               (5 "fri" "金")
               (6 "sat" "土"))
             alist)
      (let* ((en0 (cadr spec))
             (en1 (list en0 (capitalize en0) (upcase en0)))
             (en2 (append en1 (mapcar (lambda (s) (concat s ".")) en1)))
             (ja0 (nth 2 spec))
             (ja1 (list ja0 (concat ja0 "曜") (concat ja0 "曜日")))
             (re (regexp-opt (append en2 ja1))))
         (setq alist (cons (cons (car spec) (concat "^" re "$")) alist))))))

(defun my-dow-check-in-message ()
  "Check the day of week following date in message."
  (goto-char (mew-header-end))
  (let ((ol (make-overlay 0 0)))
    (overlay-put ol 'face 'highlight)
    (unwind-protect
        (while (re-search-forward my-dow-date-regexp (point-max) t)
          (let ((mdy (mapcar (lambda (i) (string-to-number (or (match-string i) "")))
                             (list my-dow-index-month
                                   my-dow-index-day-of-month
                                   my-dow-index-year)))
                (s (match-string    my-dow-index-name))
                (b (match-beginning my-dow-index-name))
                (e (match-end       my-dow-index-name)))
            (when (= 0 (nth 2 mdy))
              (setcar (cddr mdy)
                      (string-to-number (format-time-string "%Y" (current-time)))))
            (let ((wday (calendar-day-of-week mdy)))
              (dolist (pair my-dow-day-regexp-alist)
                (when (string-match (cdr pair) s)
                  (unless (= wday (car pair))
                    (move-overlay ol b e)
                    (let ((msg (format "It is afraid of %s" (aref calendar-day-name-array wday))))
                      (when (y-or-n-p (concat msg ". Do you want to modify it?"))
                        (goto-char e)
                        (error msg)))))))))
      (delete-overlay ol))))

(add-hook 'mew-make-message-hook 'my-dow-check-in-message)