Easter in Greek and Latin (the language of the first Christians) is Pascha. It’s around the same time as the Passover. That’s why it changes every year due to the lunisolar calendar. It’s just using the old Jewish calendar. The earliest record of Easter being celebrated is from the time when pagans were the ones persecuting Christians.

We don’t know for certain that Jesus wasn’t born in December.

The reason lies within Jewish superstition - that a prophet/holy man died on the anniversary of their conception. Someone, likely a century later, reckoned that Jesus died on the 25th of March (we reckon now that it was actually the 3rd of April) so this became the Feast of the Annunciation (conception). So a cycle was created where the day of the death happened on the same as the conception. Another factor is this was also traditionally believed to be the date of the world’s creation.

In fact, in the UK it was this date that was used to demarcate new years. So traditionally many people still commence and terminate land contracts on “Lady day” (the 25th of March) and the tax year begins on the 6th of April (today) which is the 25th of March on the Julian Calendar.

So simply add 9 months - you have the 25th of December.

As for the saints - most saints commenorated in Christianity are generally Biblical figures or early Christians such as St Patrick or St Nicholas