If the Pagans had truly converted, they would no longer need or want to practice their old rituals. But continue to practice them they did, so the Catholic Church had to weave a tissue-thin veil of lies^H^H^^H^H dogma around these practices in order to claim conversion even when there was none. And it wasn't limited just to the annual festivals, such as Spring, Harvest or Midwinter. Even the greater beliefs such as the first two commandments were subject to pragmatic compromise by the Church.
The Old Gods became "Saints". Of course, there is only one God, but you can have as many Saints as you like. They can even have their own individual festival days and spheres of responsibility. But, we will not refer to them even as demi-gods! No, no, no! In this way, you could still make prayers, petitions and even offerings to them just as you could with the Old Gods (no dead animals, please). And all without violating the first commandment. Isn't that convenient? This had the wonderful side-effect that the old idols no longer represented Gods as such, so they became mere statues or "icons" and thus, the second commandment was also safe. After all, praying at my little shrine to Saint Snigglefritz was really just asking him to talk to the One True God on my behalf, as I am but a worm in His presence and thoroughly unworthy of direct contact with Him myself.
But it's not just Catholicism - All religions suck.