In 3rd century Rome, a pagan feast was introduced for December 25: sol invictus, “the invincible sun,” to celebrate the lengthening days after the winter solstice on the 21st. This feast was gradually co-opted by Christians to celebrate the Lord’s nativity.
The Season of Christmas officially ends with the feast of the Baptism of the Lord.
The season of Christmas is made up of a number of individual feast days. Generally on feast days, each reading stands alone and should be interpreted through the lens of the liturgical season and feast. This is different than Ordinary Time when the Old Testament and Gospel readings are closely connected and help interpret each other.
Also during this season, the historical and cultural context of the readings matters far less when interpreting the passage than does the season itself. We can look at historical and cultural settings but these are not the primary driver for interpretation.