The question of this post's title might seem totally irrelevant, since it seems that most of America skipped Thanksgiving and jumped right on to Christmas within minutes of the end of trick or treating hours on Halloween! But I think the question is relevant.
There are at least two possible objections to observing Thanksgiving: 1) Animal rights, and 2) the plight of American Indians.
The first objection, I suppose, would look like this: Our food production system is one of intensely cruel factory farming. Pigs, Cows, Chickens, and other livestock are treated so inhumanely, that is sickening. In light of this, can we really contribute to a holiday that asks us to consume so much animal product, thus supporting this cruel system?
That objection is rather easy to answer. We can, of course, have a vegan, vegetarian, or free-range & small family farm version of thanksgiving.
The second objection is more serious. Thanksgiving celebrates the founding of this county, symbolically at least. But was not this country, in part, founded by stealing land from American Indians, killing them off, and generally treating them with inhumane cruelty and treachery? Yes. Sadly it was.
Despite this, I don't think Thanksgiving has ever been about this tragic and sorrowful fact. It's simply a time for families and friends to gather together and be thankful. It need not, and I think for almost no one is, about how this nation wronged the American Indians.
Thanksgiving is a time to celebrate and rejoice in what we have, to express a profound gratitude for life and living; for friends, family, and other loved ones. It is the start of the Holiday season; the one time of year left in this country when we actually slow down our mechanical routines a little bit and celebrate what really matters in our lives.
So by all means celebrate tomorrow! Eat your Turkey (or Turkey substitute) and mashed potatoes, watch football, say grace, and retire for the evening comfortably full and happy. Despite the struggles in life, there is always something to be profoundly grateful for.