When is Taily Day and what is it about?

April 1st -- In Scotland, for instance, April Fool's Day is devoted to spoofs involving the buttocks and as such is called Taily Day. The butts of these jokes are known as April 'Gowk', another name for cuckoo bird. The origins of the "Kick Me" sign can be traced back to the Scottish observance.

http://www.april-fools.us/history-april-fools.htm

Now, while all this fun was being had in Europe, the Scots were developing their own April Fool's tradition, called "April Gowk." This tradition relates to the arrival to Scotland of the cuckoo, emblem of simpletons, comes, so in Scotland the victim is called a gowk (cuckoo). Now, what's really interesting about April Fools Day in Scotland is that it is actually two days. The second day of hilarity is called Taily Day, a day solely dedicated to pranks involving the buttocks. Taily Day's gift to posterior posterity is the ever-hilarious "Kick Me" sign.

http://members.tripod.com/scott_michaud/HD-fools-of-April.html