Actually, not being American, I've never really understood what Thanksgiving was celebrating (apart from eating too much turkey :)).

Is it:

a) The Pilgrim Fathers actually getting to America without drowning etc.

b) ... getting to somewhere that wasn't British/French/whatever direct rule.

c) General thanks for whatever you can think of (family, health, etc)

d) Just an excuse for a holiday...

e) Something else...