The OT has been superseded by the NT.Trying to show people that all that in the OT was the foundation of all the NT instead of truncating it off. And the NT is by extension what the OT was about. If one does not understand OT, then they worship Jesus in a vacuum. I would not be so quick either to judge those who want to worship on Sabbath or Feast days of the Lord as if they are doing it for salvation (they are not). They mean something for the future and are biblically based. And yet the same would change the Sabbath to Sunday (don't know where that came from) and keep Christmas and Easter and Lents (they have no biblical basis or story behind them) and the like not realizing they are doing the very same thing they are accusing the Sabbath folks of doing. So for those who are quick to quote Colossians, I'll quote one, too:
But now after you have known God, or rather are known by God, how is it that you turn again to the weak and beggarly elements, to which you desire again to be in bondage? You observe days and months and seasons and years.
I am afraid for you, lest I have labored for you in vain. (Galatians 4)
So if that's for sacrifice by the Levitical system, which is no longer necessary, it applies all the more so to newly invented or continuing old holidays with basis in paganism. That's all the more so because Paul calls them beggarly elements (i.e. detestable fundamentals like in old times before Christ). So they were using these beggarly elements to celebrate, worship or whatever - not reference to the times themselves.
It boils down to this: can you tell the difference between someone who observes Sabbaths, feasts and festivals the OT way, sacrifices and all, rather than worships on those days New Testament style? Clue: Nobody brings a lamb to sacrifice on the altar for sins etc etc. Instead we celebrate the Lord's Supper - the NT way for that feast. I have not seen a demonstration that you do know the difference.
Why wear old clothes when you have new ones in the closet ?