I'll start with an easy one - the holidays aren't that important. You may think that Christians should follow some of the Jewish customs (e.g. Passover) but there's nothing in the Bible forbidding extra holidays that I know of.
In the bigger question of science versus religion, I have to say that on its own turf science has a pretty good record - and faith hasn't done so well. So if your faith comes into conflict with science then you really do need to question whether your faith is misplaced. The better established a scientific conclusion the harder you need to question. That the Earth is a lot more than 10,000 years old is very solid. How life originated isn't known scientifically (and may not even be knowable scientifically, although we can reasonably expect that science will eventually come up with plausible scenarios that fit the evidence we do have). That God did or did not subtly guide the course of evolution is something that we just can't investigate.
So to be honest if your brand of Christianity comes into conflict with well-established science I think you would be well-advised to at least consider more liberal versions that do not have the same conflicts.
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