the origins of Christmas being celebrated on december 25 are fuzzy, but date back to the same place all interesting things come from, namely, the pagans.
every source i looked up gave me different information, so this is a synthesized cornocopia of it all.
way back in the day, the pagans noticed that december 21-22 had the least amount of daylight in the year. today we know that it's because the sun burns out, but they didn't know about astrophysics and so thought that it was because the sun god had died.
thus, when it began to get lighter, they celebrated the rebirth of the sun god [that such a return to life was celebrated three days later is interesting in itself].
and so december 25 was the day of parties.
pagans like to throw parties, but i think they were different than the 104 apple beer parties [and i want an apple beer, for the record].
as the romans came about a-conquering, they learned that adopting conquered peoples' traditions helped keep them from becoming unruly, and so changed sun-god-day to saturnalia, named after saturn. gifts were given, people sang naked in the streets [later, clothes were added and carols were substituted for riotous vulgarities, but the general idea remains], and indulging of all the physical appetites commenced all around. in fact, roman law ceased to exist december 17-25. no law. no courts. you could kick people in the shins all you wanted.
this worked for the pagans, this worked for the romans; everyone was happy except for the poor people who were sacrificed on this day in hopes of destroying the lord of misrule, or the forces of darkness.
it never really worked.
in the 4th century, some say 336, mr. constantine decided that he could gain more converts to Christianity if he let the pagans and the romans celebrate saturnalia. the catch he faced was that this pre-mardi gras violated almost anything taught in any testament available at that time. so he did the best he could come up with and declared that the last day of the bedlam be in honor of the birth of Christ.
and so it was.
this changed and morphed over the centuries, and most of the customs we have today pre-date the birth of the Saviour and again come from pagan traditions: trees, holly, and all that.
as a side note, the puritans were aware of the profane origins of the holiday and banned it in massachusetts between 1659 and 1681.
i've heard that on the morning of April 6, the sunlight shines directly through the small circle of glass in the celestial room of the timpanogos temple.