Often understated in the Thanksgiving festivities, with its focus on feasting, is gratitude.
In some sense that may make it the most significant holiday.
I was brought up Catholic, but my spirituality today has no direct connection with organized religion. Haven't pursued other faiths, either on an intellectual or personal basis. Philosophically an agnostic, but in day to day life rely on a personal God who can give me serenity and strength.
Then again, gratitude (or any other virtue) should not be restricted to just a single day, but a reminder does help.
I'm a Thanksgiving missionary. I think Canada has a version of Thanksgiving, they'll be my first target, get them in line with US dates and customs.