"pagans practiced all the rituals that were eventually co-opted as holy days"
Although that statement is not totally true, even it was, my response would be, "So?"
Take the Christmas tree as an example. Having a Christmas tree is not Biblical. It originates in a German pagan festival. But so what?
When most people look at a Christmas tree what comes to mind? Christmas and the birth of Christ our Savior. Do people think about it as whatever it represented in the pagan festival? No. Is any of the original pagan meaning still attached to a Christmas tree? No.
Christianity is unique in that it does not require that you give up your culture (except when something in that culture opposes God--such as a culture in which multiple Gods were worshipped. In that case the acceptance of multiple gods, but not the complete culture, must be given up.) Christianity embraces and celebrates the diversity of cultures. There is nothing wrong with taking something from a culture, such as a Christmas tree, and using it in a way the honors and celebrates our Lord God.