Odds are strong that on the morning of March 4th, all over Britain and Ireland many cooks will wake up with the bizarre thought, "Oh no, I forgot to buy lemons!" This is because that day is Shrove Tuesday, the day before Ash Wednesday -- a day also known in Britain and Ireland as Pancake Tuesday.
This holiday is yet another holdover from the days when the fast imposed on European Christians during the penitential season of Lent was a "hard fast". This fast's rules required its observers not only to eat much less than they usually did, but to eat a much sparser diet -- one that completely omitted meat and other rich foods such as oil, eggs and butter. The householder therefore had to use up those foods before midnight on Ash Wednesday, the official beginning of the Lenten season.
All the great pre-Lenten festivals (of which the North and South American celebrations of Mardi Gras, "Fat Tuesday", are descendants), contain aspects of celebration that deal with this basic problem: what's the careful householder to do with all the eggs and oil and butter and so forth in the cupboard? You can't just throw them out. Therefore you have to eat them, and in a hurry.
There are several ways to do this, and different regional cultures across Europe handle the problem in different ways. Some countries like to concentrate on the oil, going in heavily for deep-fried pastries like grosti and chruscik that now routinely turn up as part of the pre-Lenten Carnival tradition. (For more info on this, see also the wonderful Fried Doughs Worldwide web page.) Others get serious about the butter: in Russia they celebrate Maslenitsa -- not just a single high-fat day, but a Butter Week, during which the traditional Russian blini pancakes get seriously soaked with melted butter, along with (it seems) just about everything else. In Ireland and Great Britain, though, the emphasis seems to be more on getting rid of the milk and eggs, in the form of pancakes.
However, the pancake in question isn't anything like the traditional North American pancake that appears in breakfast stacks all over the US and Canada. The Shrove Tuesday pancake is thin and nearly as wide as the average frying pan, more like the French crêpe than anything else. (And there may be connections to the crêpe, for the French also celebrate a pre-Lenten pancake day on the old Church holiday of Candlemas, the former Feast of the Presentation, commemorating the first time the Christ Child was brought to Temple six weeks after his birth.)
The Pancake Tuesday pancake is traditionally quickly cooked and sometimes tossed -- and there are famous connections between the day and the art of pancake-tossing, especially the famous Pancake Race which has been held yearly on Shrove Tuesday in Olney, Buckinghamshire since 1445: check out its video.
Anyway, after the cooking and tossing (and optional racing...), each pancake is rolled, laid side by side with its fellows on a plate, sprinkled with lemon juice, and dusted with confectioners' sugar / icing sugar (or granulated sugar, in older versions of the recipe).
The lemon juice, incidentally, lies at the core of just about the only attempt so far to commercialize this holiday. One particular firm (now owned by a conglomerate) has for some years attempted to rebrand Pancake Day as "X Lemon Juice Day", this being about the only time of year that there's any kind of rush on their brand of pre-squeezed lemon juice in its traditional plastic squeezy lemon. (See also Ian's trenchant comments on the subject.) Fresh lemon juice works much better.