Mother's Day is celebrated on different days in different regions of the world. The day that over 80 countries use to honour the influence of motherhood in society is the second Sunday of May, which is May 12th. While Batman can't celebrate the day with his mother, he can instead bring Calendar Man a card or present tomorrow. Calendar Man's comforting story will count towards these achievements in Batman: Arkham City (Xbox 360)
and Batman: Arkham City (PC)
Don't forget now, or next time he'll tell your father and make sure that you remember the next time that you're due to visit!