You're right in that most traditional Christian holiday have pagan roots. However, it's incorrect to think that the bible endorses or sanctions their "Christianization". There's a fundamental change in a person when they become Christian. Their old self is no more.
2Co 5:17 Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.
Of course baggage is brought into the Christ relationship, but the goal is to disregard it. Get rid of it.
There is however no biblical precedent for taking pagan customs and making them Christian.
In point of fact there are Christian holy days. The same holy days that our saviour Jesus Christ observed while incarnate. These are listed in Leviticus 23.
Traditional (as opposed to biblical) Christianity abandoned the holy days of the Lord and substituted holidays of their own.
There is no biblical precedent for many things; since when does that make a prohibition?
People like days off. And people like to relax with friends and family. There’s a reason why most culture (secular and not) generally find a reason to have a party and a day off every quarter. America doesn’t do so hot with Q1 but we make up for that with lots of days off in Q4. And people changing religions like to keep some familiar ground, changing religions is a big thing kind of scary, nice to keep some of the same days off. It’s superficial but for most people holidays really are superficial, they’ve lost their holy day origins and are a day off.