One man considers one day more sacred than another; another man considers every day alike. Each one should be fully convinced in his own mind. He who regards one day as special, does so to the Lord. He who eats meat, eats to the Lord, for he gives thanks to God; and he who abstains, does so to the Lord and gives thanks to God. For none of us lives to himself alone and none of us dies to himself alone. If we live, we live to the Lord; and if we die, we die to the Lord. So, whether we live or die, we belong to the Lord. - Romans 14:4-8
If you read all of Romans 14 in context there are a number of things that should leap out at you.
1. The focus of the chapter is eating and drinking practices. Approximately 75% of the verses directly mention eating or drinking and the rest of the verses are support for this central theme. Therefore the "days" mentioned somehow pertain to eating and drinking.
2. There is no mention of the Lord's Holy days (greek heorte). Therefore Paul or other early Christians never took this to mean that they are free to ignore the days that the Lord created holy for ones of their own choosing.
Every holiday we have a similar thread. I think it’s becoming a tradition.