The word "holiday" comes from "holy day". This is supposed to be a religious concept. The purpose of holidays is to be shared experience for a religious community to help bind the community together. This means that you should keep holidays based on which religious community you want to be bound to.
The Old Testament specifies a number of holidays. But these holidays were meant for the Israelites as a community. If you are not part of an Old Testament community, but just follow the Old Testament as an individual, then these Old Testament holidays have little value.
Instead, you should keep the holidays of the religious community that you chose to associate with. You don't have to keep all the holidays or keep them exactly as the religion specifies, but you should at least partially respect the major holidays. And try not to publicly violate the rules of the holidays.