I don't think it is odd at all. In a choice between "our religion can't gain the membership to survive" vs "ok, we can bend the structure of our religion a little to make it more palatable for this huge amount of people", I think the choice is obvious.

You might find it odd that Christians think they came up with Easter (or Christmas, or the Epiphany...) though.

With Easter, unlike Christmas, there isn't the same doubt about which time of year it was as it was around the time of the (much) older Jewish Passover celebration.

A lot of these things come from folk practices that the Church would permit but then direct towards something acceptable.