What is the origin of pancake day?
Pancake Day is Shrove Tuesday. In the Christian calendar, Shrove Tuesday is the last day before Lent, the 40-day period of fasting and self-denial before Easter.
The custom of a feast followed by fasting has very ancient roots; in Christianity, 'Shrove' Tuesday is so called because people should have confessed their sins and been absolved or 'shriven' by a priest, so they will be 'purified' in preparation for the religious month of Lent. In Lent the custom was, or is for some, to give up a number of rich foods for 40 days. This would include sweet things and dairy products among others, so one of the reasons behind Shrove Tuesday was to use up the milk, eggs etc that you wouldn't be eating again until Easter - so pancakes were an obvious dish.
Nowadays most people in Western countries enjoy a pancake on Shrove Tuesday whether they have any religious beliefs or not. Fewer people fast for Lent, but it is still common to 'give up something for Lent' such as chocolate or alcohol. You can learn more about these customs here.