Wishing someone a "Happy Thanksgiving!" can have two very different meanings.
The greeting may simply convey hope that the greeting's recipient will enjoy a break in her/his ordinary routine. Implied in that wish may be thoughts of extra time with family or friends, a sumptuous meal, and watching sports or a parade on television.
A more thoughtful interpretation of "Happy Thanksgiving!" is that we should be grateful. Gratitude requires both something for which to be grateful as well as someone to whom to be grateful. Prior Ethical Musings' Thanksgiving posts (e.g., in 2013 and 2014) have emphasized the importance of not blithely regarding all good things as God's blessings upon us. That perspective falsely implies that God frowns upon, or has even cursed, persons who do not enjoy those blessings. Instead, true gratitude focuses on those gifts, such as love and inner strength, that we truly receive from another. The giver may be God or another human.
Happy Thanksgiving! And may you take time to number the gifts for which you are thankful and give thanks to the ones who gave you those gifts.