The 22nd annual Head of the Charles regatta kicks off tomorrow from 8:45 a.m. to 5 p.m. on the Charles River.

So what, you say?

Well, for those of you who:

.(a) attended prep school

.(b) lived on the Eastern Seaboard

.(c) have discovered the illustrious joys of rowing, or

.(d) have ever been in Cambridge on this particular October weekend, the opening paragraph should be self-explanatory.

You know, The Head. It's Tomorrow.

So if you're already in the know, get moving. Go buy the beer. Go arrange the after-Head date. Go call your prep school friends and arrange for them to crash on your floor.

But to help out those who know nothing about the Head--and to refresh the memories of those who partied too hard last year--here's some background on tomorrow's happening.

The Head is a three-mile upstream race featuring 780 boats and 3500 competitors in 14 different events, making it the country's largest single-day rowing event.

Boats will be taking off from the Boston University boat house at 10-15 second intervals and finishing near the Metropolitan District Commission's Herter Center.

Since its creation in 1964, under the guidance of Northeastern rowing Coach Ernie Arlet, the Head has grown from an unscheduled racing event featuring 100 boats into the nation's premier regatta.

"It was luck," explains Ken Lynch, who has been a member of the Head of the Charles Management Committee since the event's inception. "In 1964 it was the only regatta held during this time of year. The colleges liked the day, there was good weather, and it was well-organized."

The regatta has grown tremendously in both size and reputation since its humble beginnings 22 years ago.