Once there were some Native Americans, and they had this harvest festival. Then they made some new friends who had something to celebrate - and voilà!
The world is full of syncretic traditions. These are traditions that once belonged to separate groups that then become shared or combined. What fascinates me is the various ways in which the combinations result in the original traditions being reinterpreted and taking on different meanings.
Remember Zeus, and how he beat Kronus and then decided to share power with his brothers and sisters? Remember how many wives he had, and how jealous Hera was (even though she wasn't his first wife)? Once I heard it explained that Zeus' wives explained how the Greek pantheon took on the religions surrounding it. Female deities of conquered peoples became "wives" of Zeus, thus giving them a place in the mythology as a whole. Interesting enough that I'm tempted to go research it...
Have you ever heard that the population of Japan is about 75% Buddhist and 75% Shinto (not precise figures)? Well, Buddhism is highly syncretic, and so lots of Shinto gods have been integrated into its system; at the same time, many Japanese believe in both religions at once. The two are not mutually exclusive. Coming from the Judeo-Christian background as I do, I found this surprising and fascinating when I first learned of it.
Christianity has done some conquering in its time, and some reinterpreting. I think immediately of Halloween and the dark flavor that Christianity laid over it - but also of the timing of Christmas, which so closely matches the time of the winter solstice.
When you're doing your worldbuilding, consider the religious history of your world. If there are two or more conflicting traditions, don't make it too simple - consider how they interpret one another and where they just might overlap. Also consider that a religion that denies the validity of all others is not the only option, even in our world. You might just find a way to deepen yours in a fascinating and unexpected way.