Origin of holiday

Although no one knows on which date Jesus was born, Christians have favored December 25 since ancient times. It is the date on which the Romans marked the winter solstice and it is nine months following the Festival of Annunciation (March 25). In ancient and early Medieval times, Christmas was either a minor feast, or not celebrated at all.

Around 220, the theologian Tertullian declared that Jesus died on March 25, AD 29. Although this is not a plausible date for the crucifixion, it does suggest that March 25 had significance for the church even before it was used as a basis to calculate Christmas. Modern scholars favor a crucifixion date of April 3, AD 33 (also the date of a partial lunar eclipse).^[2] (These are Julian calendar dates. Subtract two days for a Gregorian date.)

By 240, a list of significant events was being assigned to March 25, partly because it was believed to be the date of the vernal equinox. These events include creation, the fall of Adam, and, most relevantly, the Incarnation.^[3] The view that the Incarnation occurred on the same date as crucifixion is consistent with a Jewish belief that prophets died at an "integral age," either an anniversary of their birth or of their conception.^[4][5]

Aside from being nine months later than Annunciation, December 25 is also the date the Romans marked the winter solstice, which they referred to as bruma. For this reason, some have suggested the opposite of the theory outlined above, i.e. that the date of Christmas was chosen to be the same as that of the solstice and that the date of Annunciation was calculated on this basis. (The Julian calendar was originally only one day off, with the solstice falling on December 24 in 45 BC. Due to calendar slippage, the date of the astronomical solstice has moved back so that it now falls on either December 21 or December 22).

Source wikipedia 

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