History and traditions of dating
A Mass service (which is sometimes called Communion or Eucharist) is where Christians remember that Jesus died for us and then came back to life.
The 'Christ-Mass' service was the only one that was allowed to take place after sunset (and before sunrise the next day), so people had it at Midnight!
But there are records of early Christians connecting 14th Nisan to 25th March and so the 25th December go back to around 200!
The Jewish festival of Lights, Hanukkah starts on the 25th of Kislev (the month in the Jewish calendar that occurs at about the same time as December).
The early Christians certainly had many arguments as to when it should be celebrated!
But soon people wanted a separate day to celebrate his birth.In the pagan religion of Mithraism, the holy day was Sunday and is where get that word from!The Roman emperor Aurelian created 'Sol Invictus' in 274.The Gregorian calendar is more accurate that the Roman calendar which had too many days in a year!When the switch was made 10 days were lost, so that the day that followed the 4th October 1582 was 15th October 1582.