Why is Christmas on the 25 December and not 1st January?

Answer 1

Because that was the date of the Pagan festival, called Winter Solstice.

Christianity was losing the fight against Paganism. So instead of sending troops to kill the Pagans, they "stole" the Pagan festivals in an attempt to convert them.

The church stole Winter Solstice and renamed it "Christ's Mass", claiming it was the day Jesus was born, despite Jesus's apparent date of birth never being mentioned in The Bible.

Christmas never was an original celebration, it was a festival that was stolen from the Pagans and re-invented.