...is because nowhere in the bible, new testament or old and generally believed to be the Word of God (emphasis, mine), is it mentioned in terms of being a religious festival or occasion that was required to be observed or celebrated in any special way or in any other context whatsoever.
So, how did it come about, in any case? Well, here are some historical snippets of interest, courtesy of Google Search, Wikipedia and History.com, to name a few.
i) Christmas literally means 'mass for the Christ', the day on
which Christians celebrate the birth of Jesus. The date is
quite arbitrary. It was chosen by Pope Leo I to coincide
with the pagan festival of Saturnalia in which the Romans
worshipped Saturn, the sun god.
ii) The first recorded date of Christmas being celebrated on
December 25th was in 336, during the time of the Roman
emperor Constantine, the first Christian Roman emperor.
A few years later, Pope Julius I officially declared that the
birth of Jesus would be celebrated on 25th December.
iii) The 'History Of Religions' hypothesis suggest that the
Church selected December 25th so as to appropriate
festivities held by the Romans in honor of the sun god Sol
Invictus. This feast was established by the Roman
emperor Aurelian in 274. An explicit expression of this
theory appears in an annotation of uncertain date added
to a manuscript of a work by 12th-century Syrian bishop
Jacob Bar-Salibi. The scribe who added it wrote: "It was a
custom of the Pagans to celebrate on the same 25th
December the birthday of the Sun, during which they
kindled lights as a token of festivity. In these solemnities
and revelries Christians also took part. Accordingly, when
the doctors of the Church perceived that the Christians
had a leaning towards this festival, they took counsel and
resolved that the true Nativity should be solemnized on
that day."
iv) The English term Christmas ('mass on Christ's day') is of
fairly recent origin. The earlier term Yule may have been
derived from the Germanic jōl or the Anglo-Saxon geōl
which referred to the feast of the winter solstice.
v) In the early years of Christianity, Easter was the main
holiday; the birth of Jesus was not celebrated. In the
fourth century, church officials decided to institute the
birth of Jesus as a holiday. Unfortunately, the bible does
not mention a date for his birth. Although some evidence
suggests that his birth may have occurred in the spring
(after all, why would shepherds be herding in the middle
of winter?), Pope Julius I chose December 25th. It is
commonly believed the church chose this date in an effort
to adopt and absorb the traditions of the pagan Saturnalia
festival. By holding Christmas at the same time as
traditional winter solstice festivals, church leaders
increased the chances that Christmas would be popularly
embraced.
Messy origins it appears to have had, if the information available on the Internet was to be believed, but one common thread appears to link it to a pagan festival celebrating the birth or rebirth of the sun god and the winter solstice, a mid-winter event marking the reversal of the gradual lengthening of nights and shortening of days, or the return of longer days and shorter nights following a period of shorter days and longer nights in other words.
On second thoughts, perhaps I should resume celebrating Christmas once again. After all, the more than two billion people doing it can't be wrong, can they?