This is a fairly simple achievement. You simply have to interact with Calendar Man on a specific date each month. These are holidays.
Go to the Courthouse through the back entrance and go down the stairs If you look to the wall beside the Calendar Man you will see a calendar with dates circled.
On your PC change your PC date to the specific date needed. The great thing here is you can literally minimize your game, change the date, maximize the game back up and talk to the calendar man and he will recognize the date change making this super simple. This makes it quite quick. The dates needed are as follows:
New Years Day - Jan 1st
Valentines Day - Feb 14th
St. Patrick's Day - March 17th
April Fool's Day - April 1st
Mothers Day - May 12th
Father's Day - June 16th
Independence Day - July 4th
Feast of St. Roch - August 16th
Halloween - October 31st
U.S Thanksgiving - November 28th
Christmas - December 25th
If you have any issues switching the dates, give it about 20 seconds before talking to Calendar man and if that fails, simply leave and re-enter the courthouse and that will force the refresh as well as update the Calendar on the wall and save your game. I found that the game would glitch after 2 or 3 date changes forcing me to leave the courthouse.