Why do churches celebrate pagan holidays? God gave us a set of holy days. Also read Colossians 2:8 God is not about tradition. Also read Deut.12:30-32. Also Jeremiah 10:1-3.So many churches teach to follow the Bible but they add worldly tradition. In Rev 22:18 it says do not take away or add to this book.
I can’t answer for most churches, why they do things. You would have to ask them. Instead you are asking someone who does not celebrate any particular religious holiday. God did give the Jews certain holidays, but does not require them of Christians. In fact, he tells Christians not to judge a person based on whether or not he keeps any holiday. “Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days.” (Colossians 2:16) So if somebody chooses to keep Passover, or chooses not to keep Passover, I am not to judge him. If someone celebrates Christmas or Easter, or chooses not to, I am not to judge him. If he demands that I celebrate them, then he is wrong, based on the same scripture. But if he chooses to celebrate on his own, that has nothing to do with me. For a more detailed explanation, please see my article at Do We Celebrate?
I do have some further comments, though, about the passages you mentioned. Revelation 22:18-19 says, “For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book: And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and [from] the things which are written in this book.” He specifies that he is talking about the prophecies in the book of the Revelation. This passage says nothing about the rest of the Bible.
Deuteronomy 12:29-32 prohibits the Jews from following the practices of a limited number of nations. He says nothing about their holidays. What he tells them is not to follow the worship of other gods that these nations practiced, and specifically not to sacrifice their children to the gods. Jeremiah 10:1-6 (I’m not sure why you stopped at verse 3) is also specifically about making and worshipping idols. Although a general principle may be taken, it would be against idolatry.
Colossians 2:8 does teach against binding the tradition of men. On the other hand, Paul tells the Thessalonians to stay away from one who doesn’t walk “after the tradition which he received of us.” (2 Thessalonians 3:6) There are apparently some traditions that we may follow.
Much of what any church does is tradition. The songs they sing, the frequency of taking the Lord’s Supper, even meeting in a “church building” are all traditions. The point at which it becomes wrong is when they bind it on people as a requirement rather than an option.
I agree with you that some holidays may be based on pagan practices or dates. In many cases, though, people are unaware of their pagan backgrounds and are thus not worshipping in pagan ways. The bottom line is that the celebration of any holy day is merely traditional for a Christian.