it's pretty much common sense squidly.
Do you think anyone actually noted down the exact date that 'Jesus' was born? (in fact is it even mentioned in the bible?)
When one religion takes over another, they absorb a lot of bits and pieces in order to make the change more palatable to the converted.
For winter solstice read christmas, spring solstice read easter etc etc.
I'd advise reading
this if you want to learn a bit more about it.