As an atheist, there are several holidays that I do not celebrate. A few of our holidays come from religious, usually christian roots. ...even though they have pagan roots that are undermined by the majority religion. Actually, there are a few religious holidays that I actually do celebrate, but not the way most celebrate them. I appreciate the roots that were pre-christian in these holidays, and since my fiance is pagan, I celebrate those with her.
I always found it interesting that we have a "Federal holiday" in our country that is not secular. While most of our national holidays celebrate the forming of our nation, or events that defined our culture as a secular nation, Christmas is on that list which goes against the first amendment. ..Not to mention that it's roots were undermined in the first place.
Very many holidays that we have, have roots that most people seem to not be aware of..even the people who invented them. Groundhog day for example has pagan roots. These roots are often forgotten, undermined or just plain stolen.
examples: Christmas and Halloween.
There is also a few holidays that I would like to NOT celebrate, though I do.
Valentine's day comes to mind. Like I've mentioned before, a lot of people dont seem to know the roots of a holiday because it is well hidden behind commercialism. Since St. Valentine has the holiday named after him, I'm obviously not a person who gives this holiday much attention..however, I have a girlfriend who would probably be a little upset if she didn't get a heart shaped box of candies and a few roses on this day.
Other holidays I do not celebrate are:
Easter - resurrection of a savior who I dont beleive in.
Columbus Day - This one has nothing to do with religion..this one has to do with the principle behind it. He killed and deceived natives and thought he landed in the east..but yet we celebrate him. Never made sense.
St. Patrick's day - this one is the same as St. Valentine's day.
Mardi Gras - this one isn't really a holiday..more so a popular celebration. It's a catholic celebration, that everyone seems to like to be a part of.
To sum everything up, I observe secular holidays and the original version of religious holidays because of an appreciation of there pagan roots.