The day dedicated to commemorating the birth of Jesus Christ.
It is set on December 25th. People often confuse the origin of that date with the birth of a sun god from a pagan religion, but it is just a coincidence and not the case.
December 25th was picked as Christmas day because it is 9 months away (March 25th) from the start of Virgin Mary's pregnancy (in the sixth month of Elizabeth's pregnancy with John the Baptist, according to Luke 1:26). It's also the reason why some church decided that Christmas is on January 6th, because they believe Virgin Mary start her pregnancy on April 6th.
a time when many family members, including your great-great-aunt that you haven't seen for 12 months, all gather together to get stuff and eat food. whether you celebrate it for the religion or not, it's still nice to get presents. we're humans, we're greedy.
i hope i don't have to see my great-great-great-great-great-aunt-mary for christmas. she always makes raisin cookies that taste like raccoon fur.