“Happy Thanksgiving” is a greeting worth considering.
On the surface, it is a well-intended wish that the recipient will enjoy the holiday. After all, how can one help but be happy if one only takes time to give thanks? It makes sense. If you focus on the blessings in your life, and take time to appreciate them rather than take them for granted—or better still, focus on the good rather than the bad—you are bound to feel better about how things are going.
In a strict sense, there is an implicit imperative to “have a happy ___(holiday)___.” Does your commanding me to be happy then make it so? Not so much. Were it only so simple!