While many countries engage in the festivities surrounding St. Patrick's Day, Ireland is the country in which the holiday originated. The day marks the death of Ireland's patron saint and is marked by a national holiday in the region. In between consuming copious amounts of alcohol, players may wish to head down into the basement of Arkham City's Court House to pay a quick visit to Calendar man, somebody who you wouldn't naturally think of celebrating with. If you do, you'll earn one notch towards the 12 needed for the following achievement:
If you happen to own a disc-based copy of Batman: Arkham City (PC)
that can still be used through the Games for Windows LIVE service, you will also be able to cross off another date for the equivalent achievement in that version of the game:
Don't forget now. If you forget then Calendar Man will have a killer joke lined up for your next visit.