Romans 14:5New International Version (NIV)
5 One person considers one day more sacred than another; another considers every day alike. Each of them should be fully convinced in their own mind.
This verse has nothing to do with holy days. The greek text doesn't mention "holy" or "sacred". In context this verse is referring to eating practices. It's most likely referring to a problem in the early church where clean animals had been sacrificed in pagan worship and later sold in the meat market. The issue as that early Christians were conflicted on whether they could eat meat that had been sacrificed to idols. Some became vegetarians. The "days" were likely those days that pagans sacrificed and put their meat out in the market.
Prov. 14:12 There is a way which seemeth right unto a man, but the end thereof are the ways of death.