It's open ended.
Each of us decide by ourselves what to be thankful for. There's no central authority (Minister of Thanks) that tells us what's on order for the day, rather its 300 Million individual decisions made on our own. That's very American, when you think about it.
Kris Kristofferson wrote a cracker of a song for this, from his 2006 album This Old Road. It's pretty old school country, from one of the masters of our day.
Thank You For A Life (Songwriter: Kris Kristofferson)
Thank you for a life that I'd call happy
Overlooking all that we've been through
When it comes to loving I've been lucky
Everything I am I owe to you
Thank for the little girls you gave me
Thank you for them bouncing baby boys
Thank you for the sadness
That you saved me from the madness, baby
All I'm crying now are tears of joy
Thank you for that burning sun that's rising
Golden in the air that smells so sweet
Thank you for that empty far horizon
That opens to a new eternity