Claire, our American Thanksgiving finds its origins in your Harvest Home celebrations! It is observed later here for various reasons, but the actual customs and traditions derive from the observance in your homeland!!
I'm kind of tired at the moment, but maybe I'll give some more info later. I haven't looked at the wiki article yet (sorry, Kathy, I am still skeptical about wiki articles; call me an old-fashioned scholar, haha!) I've spent a significant amount of time researching and writing about this subject. Either that, or maybe I'll post something on my blog. And you are on my buddy list for my blog, so I'll let you know if I do.
It's a national day of thanks (whatever one is thankful for). It was made a national holiday by President Lincoln. In WWII, congress made it the fourth Thurdsay in November. Before that, it was the last Thursday in November. This is all from memory, mind you. It's in November because (and I'm guessing here) because that's when the harvest is brought in before winter.
It's a day of feasting with family and/or friends. Turkey is traditional (athough some folks do a ham) with mashed potatoes, sweet potatoes, stuffing, veggies and pies for desert (traditionally apple and pumpkin). Although my uncle's Italian mother made spaghetti in addition to the usual foods. The foods are traditional late autumn harvest foods. Although you can get them year round now.
It's a secular, non-demoniational holiday. Basically, anyone and everyone is invited to celebrate this day of giving thanks for what we have in our lives.