Odd. I heard it was set when it was to offset the Pagan celebration, much as Hanukkah became a big deal so Jewish kids wouldn’t feel left out during the Christmas season.
There are a LOT of revisionist theories about why Constantine chose the dates he did. I suspect that the main reason he chose them was because people were partying on those dates anyway so there was no need to disrupt their lives.
But the codification of Paganism was also to eradicate any connection to OBTW Yah'shua celebrated Hanukkah John 10:22.If one needed a birthday for Yah'shua, a clear reading of Luke 1
shalom b'shem Yah'shua
would lead you to the Feast of Tabernacles in the Fall on the 15th of Tishrei.
Judaism as well and replace it with Paganism.