A question
 

Does it make sense for an atheist to bid adieu? Say goodbye? Celebrate a holiday?

While not officially an atheist, I don't exactly understand your question.

Say GoodBye, see you later ... why not.

Celebrate a holiday, presidents day (OK teasing).

Most religous holidays have become so commericialized they bare little resembalance to the days they are meant to be.
Look at Xmas.
I don't really celebrate Xmas (the Wife drags me kicking and screaming through the process) as Xmas, except as a family get together.
Which is always something to celebrate, family and friends.

Adieu -- with God.
Adios (same).
Good-bye -- God be with ye.
Holiday -- Holy Day.

Let's just consider it common courtesy.

I also do not or may not know your religious beliefs, and one of my religous beliefs is not to question your beliefs. They are yours and I have mine.

Do you think a gov official may invoke those terms in his official capacity?

What's the problem if I celebrate Christmas or Easter, while denying the (supposed) history of the holiday?

Is it wrong for a Christian to have a holiday which is planned to occur during Saturnalia, then claim that the day represents something else?

They are part of the common language at this point, and most holidays are very commercialized.

Christmas is about presents, Easter about rabbits and candy, and Halloween candy.

4th of July is about getting drunk and blowing stuff up, and Memorial day getting drunk and the official start of summer up here in New England.

Does it make sense for a Christian to use the common English-language names for the days of the week?

That's an interesting question in it's own right, but is different in this sense: Friga, Woden, etc are all deities. Christians believe in a deity. On the other hand, an atheist referring to Woden or Friga is a different class of phenomenon.

Returning to the government official, if the President says, "Good-bye" is that different from the president saying, "Good be with you"? If the President were an atheist and said, "God be with you" then he is either being a complete hypocrite and ass, or he is just mouthing a common platitude to fill airspace. If the President is a devout theist of some sort and says, "God Be with you" then he is invoking his deity's power to influence your life. That would be a religious expression.

Which one is the greater threat to the 'separation of church and state'?

You misunderstand the concept of SEPARATION of church and state. It DOES NOT have to do with disavowment of religion by government or by people in the government.

Correct. A mere utterance of something construed religious doesn’t advance religion, and it certainly doesn’t constitute an excessive entanglement.

I would posit that in spite of their etymological origins, the meaning of these words has changed through centuries of use. They no longer carry the religious connotations that they once did.

If I referred to an atheist as a "cretin", would I be doubly insulting him since etymologically the word originally meant "Christian"?:D Sadly, I would not.

Cretin Hop
 

There's no stoppin' the Cretins from hoppin'!

Okay. You guys all seem uncharacteristically in agreement. Weird, but ok.

Now let's say I'm a gov employee and I hang out with my buddies in a public building and we pray to our favorite deity. Perhaps Woden? YHWH? It may matter, I don't know. Then some smarty-pants political fanatic comes along and gets wildly offended. Public prayer by a public employee on public property. Oh, the humanity!

Now my buddies and I laugh and laugh because ... we're atheists. Did i still violate the 1st Amendment prohibition even I didn't believe a word of the haranguing nonsense we were espousing?

Suggestion: be more cosmopolitan:

say: au revoir! or a la prochaine fois! (until next time) or do zvidanie!

celebrate festival days, or memorial days like Armistice Day and the Winter Solstice