Genesis 3:15 (NKJV) says,
“And I will put enmity
Between you and the woman,
And between your seed and her Seed;
He shall bruise your head,
And you shall bruise His heel.”

In the very beginning, God promised that a Messiah would come to defeat the devil.  2000 years ago, that promise was fulfilled when Jesus was born. So, every year we celebrate that promise of Jesus being born. Christmas is a reminder that God keeps His promises!