Christmas is the celebration as the days start to lengthen, which in the Northern Hemisphere, is in the middle of winter.
The "reason of the season" is honoring the sun, it’s death on December 21st and resurrection or rebirth on December 25th.
From the summer solstice to the winter solstice, the days become shorter and colder.
From the viewpoint of the northern hemisphere, the sun appears to be moving south and gets smaller and the days get shorter.
The shortening of the days and the approaching winter solstice symbolized death to the ancients.
So what we have here is the death of the Sun.
By December 22nd, the Sun’s death was final, it had moved south continually for 6 months, made it to it’s lowest point in the sky.
It appears that the Sun has stop moving south, it hangs there for 3 days.
During the 3 days the Sun hangs on the Southern Cross, or Crux, constellation.
And after this time on December 25th, the Sun moves 1 degree, this time north.
It’s the beginning of longer days, warmth, and Spring is not far off.
And so it was said: the Sun died on the cross, was dead for 3 days, only to be resurrected or born again.
It is the Sun’s movement back into the Northern Hemisphere, which brings Spring, life and salvation.
However, they did not celebrate the resurrection of the Sun until the spring equinox, or Easter.
This is because at the spring equinox, the Sun officially overpowers the darkness, as daytime thereafter becomes longer in duration than night, and the rebirth of spring become obvious.
Christmas, a secular midwinter holiday season, a multicultural festival with ancient Pagan history, can be celebrated by everyone.