Tomorrow is Easter Sunday. It's also April Fool's Day. While a lot of people will be celebrating the former rather than the latter, Batman is an exception to that rule. He'll make sure to visit Calendar Man at some point during the day so that they can share a joke together and cross off another of those 12 murderous dates needed for the following achievement:
Calendar Man can be found in the basement of the Solomon Wayne Courthouse. You gain access to the Courthouse near the beginning of the game. All you need to do is to talk to him on each of the 12 dates that are circled on the calendar outside his cell.
Alternatively, you can also gain this achievement offline by changing the date on your console. If you're one of the people who want to actually visit Calendar Man on every date needed, the next date to remember will be May 13th.