It's called "Pancake Day" or "Shrove Tuesday". It has a religious element, although not for me - I'm only interested in the pancakes. I made a plateful of them this evening. Search online for information about it.

In Catholic tradition (and many other branches of Christianity as well), a period known as Lent begins 40 days prior to the holiday Easter, on Ash Wednesday. During Lent, a believer is supposed to give up certain luxuries and indulgences as an act of devotion to God. Shrove Tuesday, or "Pancake Day" (also known as Mardi Gras - Fat Tuesday in French) is the last day before Lent begins. Because people are about to be abstaining from indulgences for 40 days, it is their last chance to splurge or be gluttonous. So they do. It is not a religious tradition so much as a secular tradition in response to Religious demands. Different cultures have different foods or activities that they traditionally consume or participate in on the day before Lent. But the idea is that you get it all out of your system so that you can begin fasting, or abstaining from certain vices, etc.

Pancake day is British English term for the day before the start of Lent. In other countries it's called Shrove Tuesday, Mardi Gras or Fat Tuesday. In England it's called Pancake Day because of the tradition of eating pancakes on that day. In traditionally Christian countries Lent is a period of fasting before Easter. Mardis Gras/Fat Tuesday/etc. is celebrated as sort of "the last hoorah" before having to go through a month of fasting and giving up certain foods.