People who regard 13 as unlucky - triskaidekaphobes - are especially unnerved by Friday the 13th. There is always at least one a year, often two and occasionally, 28 times since 1800, three. Today is the second and last for 1989, and time to shed some light in this dark corner of the calendar.

The portent of this particular day derives from the coincidence of two sets of superstitions landing on the same 24 hours, some involving Friday and others the number 13. One tradition holds that Eve tempted Adam with the apple on a Friday. Another says Friday was the customary day for executions in the Middle Ages. As for 13, Nordic myth tells of a banquet arranged for 12 gods at Valhalla. A 13th, Loki, crashed the party; a notorious troublemaker, he hadn't been invited. There was a fight in which Baldur, the most popular of all the gods, ended up a corpse. Christian tradition links 13 and Friday, going back to the Last Supper - Jesus and his 12 disciples. He was betrayed by one of them and crucified on a Friday.

But contrarians point out that 13 was lucky in ancient Egypt. Symbolically, it was the final step in the ladder of life. Note that there are 13 steps up the pyramid on the back of a dollar bill. (Triskaidekaphobes, on the other hand, observe that there were 13 steps up to the English gallows.) In modern times, a baker's dozen is a modest windfall - an extra, free. And what man or woman toiling a five-day workweek hasn't ever said ''T.G.I.F.''?

Have a good day.