Skydiver119 posted this which might help:
There is a glitch with the calendar app. Do you have an all day appointment on April 1, 2013? if so, alter it, make it a few hours instead of all day and March will return.
The calendar app has had issues with April 1 being the start of daylight savings time and the fact that 'all day' can mean anything from 23-25 hours, and this glitch causes March to disappear. Remove 'all day' from the equation, the calendar app is happy again and gives you back the month of March.