Excellent question! Especially since my father is a used car salesman. haha.
Anyways, I know that certain people will not celebrate things like Christmas and Easter because they are not in the Bible, and the reason for this I believe is two-fold. First of all, the Bible actually prescribes a very specific list of holidays that it requires. These are the Jewish holidays, such as Passover, Rosh Hashanah, Yom Kippur, Sukkot, and Shavuot, among some others. Within this law it is stated that no one may add nor take away from the Law.
Therefore, a strict interpretation might seem to indicate that these holidays, AND ONLY THESE HOLIDAYS should be celebrated, at least as RELIGIOUS holidays.
However, Jews do celebrate some other holidays besides these because they are believed to be "within the spirit" of the Law. For example, the Bible teaches that one should rejoice in the great things that God does in history. So, Hanukkah celebrates a miracle that God performed for the Jewish people, and is considered to be acceptable to celebrate. But it never is given the high status of the holidays which the Bible specifically describes.
On the part of Christians, I think it is noteworthy that Jesus seemed to accept the holiday of Hanukkah, even though it was post-Old Testament. His disciples celebrated it, and Jesus told them to do so.
Most Christians however do not make this connection though I think, because they are not familiar with Jewish custom, and do not realize that Jesus was referring to Hanukkah when he mentions it. (I'll get you a verse if you'd like, just ask and I can look it up).
So some Christians think that no holiday should be celebrated if it is not in the Law. Christmas and Easter and Birthdays are not mentioned therein.
Others would say that birthdays are ok because they are not religious.
Others would say that Christmas and Easter are not permissible because they derive from non-Christian holidays that they believe embraced ideas contrary to their faith.
There are many things which have become custom and the people tend to assume it is biblical, when really, it is not necessarily. Even the idea of getting married in a church or place of worship is not mentioned, and yet many would say firmly that a marriage done in a courthouse is not a marriage "in the eyes of God."
I don't know why they would hold this view except that they must be ignorant of what is in their scriptures and what is not.