Romans 14:22-23 "Hast thou faith? Have it to thyself before God. Happy is he that condemneth not himself in that thing which he alloweth. And he that doubteth is damned if he eat, because he eateth not of faith: FOR WHATSOEVER IS NOT OF FAITH IS SIN."
Paul here is referring to idol worship, which is basically the topic of this discussion, or pagan customs.
If we tried to rid ourselves of each and every pagan and idol custom or thing in our lives, we really should just go hide out in a mountain somewhere, and live like a hermit off the land, because otherwise, we will be dealing with some form of pagan idol/worship on a daily basis.
For if we follow the Julian calendar and refer to today as Monday, we are speaking of a pagan thing. It is almost virtually impossible to live in our culture without being a part of some sort of pagan symbol, idol, or custom.
That being said, if we WORSHIP anything other than the one true God, and give our allegiance, and life over to it, that is a totally different thing. As long as our FAITH is in Jesus in whatever we do, then I believe God accepts it, even as Paul said, whatsoever then is not of faith, is sin.