Pancake Day or Shrove Tuesday is on 12th of February in 2013.
It's Easter that fixes the date for Shrove Tuesday. Therefore working backwards, we have Lent with its 40 days of fasting before we get to Easter. Now Lent starts on Ash Wednesday, so the day before is when cooks use up all the 'naughty foods'. In the olden days two 'naughty foods', which are not allowed in Lent, would be butter and eggs - ideal for making pancakes. Hence Shrove Tuesday, or Pancake Day.
Curiously, when calculating Lent, Sundays don't count, hence Shrove Tuesday is actually 47 days before Easter Sunday.