| The day before Ash Wednesday, the first day of Lent. In Britain, traditionally the day on which people attended church to be shriven, or cleansed of their sins following confession. Shriving is symbolised by a thumb-mark in ash on the forehead of the confessor, put there by the minister or priest. I don't know of this happening much within the Church of England any more, but the children from the Catholic school at the end of my street (St Mary's Primary in Edinburgh) can be seen parading back from the cathedral every Shrove Tuesday with ash on their brows.
Also known as Pancake Day, because of the tradition of eating pancakes, which arises from the traditional Lenten fast, in which fat, eggs and milk can't be eaten. Conveniently, along with flour, these ingredients make pancakes, and so pancakes were made to use up the left-over forbidden foods before the fast started on Ash Wednesday. The pancake pig-out persists, though the fast for the most part doesn't.
Contrary to speculation over on the Pancake Day node, pancake races are alive and well in the UK: I've taken part in a few, when I was in primary school. The aim is simply to get to the finishing line first, without dropping your pancake, which you have to keep flipping right through the race. There are often (especially for young children) races where the pancake and pan are substituted with something flippable, and something to flip it in/ on: I've seen ping-pong bats and pieces of lino in my time... the joys of coming from semi-rural southern England are unsurpassed. | 
