Updated calendar.module:

 - Locale'd the day-of-the-week-letters. Don't worry, you won't find any "F" or "S" in your locale database, just "Friday" and "Sunday".

 - Fixed a bug with 28/20-day months. Skipping a month ahead from e.g. January 31st would have you end up on March 3rd. Same for skipping backwards.

 - Made the selected date always appear in bold.

 - Prevented all skipping into the future. This was still possible by skipping whole months.