Saturday is the Jewish sabbath.  Catholics moved it to Sunday because the gospels say Christ was raised on the 3rd day (of his death, Friday being Day One).  When the Adventists broke away they moved their sabbath to Saturday.

December 25th was a pagan Roman holiday which the Church decided to use to celebrate the birth of Christ.  Catholics do that a lot, kind of a jerk move we do when we win.  Like building our cathedral in Mexico on the ruins of the biggest Aztec temple.  Not for ourselves, you understand. For His Glory. That makes it ok to rub it in.

The Catholic Calendar isn't always literally re-enacting the past.  For instance, Lent is 40 days to make 40 days from Palm Sunday - the entrance of Christ into Jerusalem -- to Good Friday.  This reference to 40 days may not be literally exact.  But it reminds you of the 40 days of the Flood, and the 40 years the Israelites wandered in the desert. These are periods of testing ending in covenant -- like the last days of Christ leading to the Resurrection.