The definition of Thanksgiving:

thanks·giv·ing
noun /ˌTHaNGksˈgiviNG/
thanksgivings, plural

The expression of gratitude, esp. to God
– he offered prayers in thanksgiving for his safe arrival
– he described the service as a thanksgiving

(in North America) An annual national holiday marked by religious observances and a traditional meal including turkey. The holiday commemorates a harvest festival celebrated by the Pilgrims in 1621, and is held in the US on the fourth Thursday in November. A similar holiday is held in Canada, usually on the second Monday in October.