Play offline (not logged into Xbox Live or PlayStation Network as your system date is automatically set via the internet). Set the date of your console to one of the following dates: January 1st, Feburary 14th, March 17th, April 1st, May 8th, June 19th, July 4th, August 16th, September 5th, October 31st, November 24th, December 25th. Then, load a saved game file where the story progress is past rescuing Catwoman. Enter the courthouse and talk to Calendar Man in the basement. He will tell you a story about the current month's holiday and tell you to return for the next month. Save the game by leaving the building. Exit to the system settings, then manually change the date to the next holiday. Repeat the process until you have cycled through the entire calendar year. The calendar on the wall next to his cell will highlight the holiday dates for the current year and scribble out which months you have already spoken to Calendar Man about.