Calendar Man is found in the basement of the Courthouse and can be visited at the beginning of the game. Speak to Calendar Man on each of the following dates that can be found on the calendar in his cell. You must exit the building in order for the achievement progress to save. If done correctly you will notice the next time you speak to him the date is scribbled off the calender. It will take approximately 30 minutes to complete if you do it offline and simply change your console's clock to each of the following dates:
10/31/2011 - Halloween
11/24/2011 - Thanksgiving
12/25/2011 - Christmas
1/1/2012 - New Years Day
2/14/2012 - Valentines Day
3/17/2012 - St. Patrick's Day
4/1/2012 - April Fools
5/13/2012 - Mothers Day
6/17/2012 - Fathers Day
7/4/2012 - Independence Day
8/16/2012 - Feast Day of St. Roch
9/3/2012 - Labor Day