This Achievement (or Trophy for all you PS3 gamers) requires you to “Have 12 murderous dates with Calendar Man”. This involves nothing more than talking to Calendar Man on 12 different dates – one per month, in fact. Calendar Man can be found locked away in the basement of the Courthouse at the top left of the map.

Below are the dates in order since the game launched:
31/11/2011 – Halloween
24/11/2011 – Thanksgiving
25/12/2011 – Christmas Day
01/01/2012 – New Years Day
14/02/2012 – Valentine’s Day
17/03/2012 – St Patrick’s Day
01/04/2012 – April Fool’s Day
13/05/2012 – Mother’s Day
17/06/2012 – Father’s Day
04/07/2012 – U.S. Independence Day
16/08/2012 – Feast Day of St Roch
03/09/2012 – Labour Day

Please keep in mind that Thanksgiving, Mother’s Day, Father’s Day and Labour Day will be on different dates after 2012.

You could, if you’re so inclined, disconnect from Xbox Live and change your system date to match and speak to Calendar Man.