If you're an atheist, then by definition, it doesn't matter what holidays you celebrate.
If you want to be nice to your spouse, then that's most likely the over-riding concern that determines how you want to conduct your celebrations, and you'd go through the motions of 'celebrating' or at least acknowledging whatever occasions [s]he wants to celebrate. A Jewish spouse that married an atheist ought to be quite easy to please.
On the other hand an atheist can participate in any holiday he wants as long as he finds the company of the other celebrants pleasant, the food enjoyable and the opportunity to interact with people he likes worthwhile. He is not constrained to celebrate only the appropriate holidays for his peer group.
In that sense, the atheist enjoys a unique flexibility, although it may justifiably be remarked that the line between participation and celebration has become fuzzy.
Christmas and Easter
Yes, since Hanukkah is a Jewish holiday.
Gentiles aren't Jewish. Hanukah is a Jewish holiday.
Channukah can start any time from the end of November to the end of December.
Muslims don't celebrate Kwanzah. That's a Jewish holiday.
The people who celebrate Hanukah are Jewish instead of Christian. Christmas is a Christian holiday, and Hanukah is the Jewish holiday that falls closest to Christmas.
Hanukah.
Kwanzaa is a Jewish holiday celebrated in december.
jewish new year
A gentile does not typically celebrate Hanukkah, as it is a Jewish holiday that commemorates the rededication of the Second Temple in Jerusalem.
Hanukkah is a Jewish celebration with no connection to Buddhism.
Hanukkah falls on the 25th of Kislev, which may coincide with November or December.