Gould,
Thanksgiving is an formal day to give thanks to God.

There you go, bracket creep already.

Thanksgiving is a day to be thankful.
For many that will mean being thankful to God. Others will be thankful to their
ancestors, their neighbors, or another diety.

When Washington proclaimed the original "Day of Thanksgiving" there was some
reference to God. But it was more akin to
"National Dairy Week", or proclaiming a single day "Superbowl Championship Team
Day".

Thanksgiving was celebrated off and on for the next 150 years. Some states
would have a Thanksgiving day, others would not.
There was no common day of observance throughout the country. I believe it was
Franklin Roosevelt who proclaimed the third Thursday in November a national Day
of Thanksgiving.

I'll keep a sharp eye peeled this next week, but I don't see many publicly
funded thanksgiving decorations and those that are out there would tend to
feature early American settlers, (pilgrims), wildlife (turkeys), and maybe even
a display of a harvest cornucopia. If there is a religion that worships
pilgrims, turkeys, and vegetables, I'm not aware of it.

Since Christmas and
Thanksgiving are Federal and State Holidays, celebrating Religious holy
days, hasn't the government officially endorsed these holy days?

Christmas was never a legal holiday in the US until 1870. President Grant
proclaimed Christmas a holiday via presidential decree. It's doubtful that this
religious day would have survived a congressional debate or judicial review.
Christmas was never envisioned as a federally recognized day by the framers of
the constitution. Until the early 1800's, it wasn't even widely observed.

Can you name a church that considers Thanksgiving a " high holy day"?

A number of days throughout the year are declared holidays. Everybody gets
Armistice Day off, whether they think peace is a good idea or not. Everybody
gets President's Day off, whether they think Washington and/or Lincoln were
good guys, or not.

Most Christmas celebrations are entirely secular. Trees, Santa Claus, holly,
drunken orgy Christmas parties, etc etc etc etc........most of the common
trappings, have nothing to do with the birth of Jesus or Christian symbolism.
Hard to make a case that Christmas, as celebrated today, is any sort of
religious holiday for most of the participants.

It is easier to make a case that when the Town of Smallville spends the public
tax money on decorations of a religious nature- (and those that depict an event
described in the Bible would certainly qualify as religious)- the government is
promoting, or endorsing, the establishment of a particular religion.