A Christmas tree.
Christmas is today! Yay!
Wait, it isn't? Oh.
Ahem. Christmas is a holiday celebrated in December based around celebrating the birth of Jesus Christ. It is the job of Father Christmas to deliver presents to the children on the night before this holiday.
In Badfic[]
Christmas is also the time that a load of annoyingly poorly written stories pop up in many fandoms on The Pit and other such websites, most of which are an excuse to either get two characters kissing under the mistletoe and/or completely rip-roaring drunk, which often leads to other more adult activities, implied or not. That or everyone plays party games, and gets drunk out of their minds. This applies whether the series is set in a place or time that would celebrate Christmas or not, as badfic writers don't care to actually research the customs of the country or time that the series is set in. Seriously, how hard is it to check Wikipedia for information on how Christmas is celebrated, or not, in other countries, and on when it started being celebrated?
What's even more annoying, is when Christmas is celebrated in continua where it logically shouldn't exist, such as the Discworld continuum, which has a celebration called Hog's Watch Night instead.
Another charge is having characters who wouldn't canonically celebrate Christmas be celebrating it anyway. This includes people whose religions don't celebrate it, such as Jewish people and Jehovah's Witnesses, and fictional races such as Bajorans.