Long before "Chrismukkah" arrived in the early 21st century, Christmas and Hanukkah celebrations were informally merged with one another. As said by Wikipedia, a Christmas celebration with a tree, songs, and gifts became a symbol of being a part of German culture for many middle-class Jewish families in the 19th century. Some Jews celebrated Christmas as a "festival of the world around us" without religious meaning, or they transferred Christmas customs to the Hanukkah festival. 

 

"Chrismukkah was named for the time, and prominently featured, in the FOX television program The O.C. (2003-2007). Show creator Josh Schwartz used the holiday (which the writers almost named "Hanimas") to depict, he later said,

"My experience as a Jewish kid from the East Coast coming to the USC (University of Southern California)... and being surrounded by all these kids from Newport Beach who were water-polo players, and these very blonde girls who only wanted to date them. I felt very much like an outsider. Even trying to talk about Hanukkah with some of them was like coming from an alien planet and talking about life there. The show is really about outsiders: Ryan was the most obvious outsider, as was Seth. The idea of a mixed (half-Jewish, half-Christian) family in Newport would also contribute to the Cohen outsider-family status. That part of their identity was always very important. Seth coined a holiday that would both celebrate and underline his outsider status in Newport. That led to Chrismukkah." (Wikipedia)  

 

"On The O.C., as a way to merge his parents' two faiths, Seth Cohen claims to have "created Chrismukkah" when he was six years old. There series included annual Chrismukkah episodes for every season of its run (The Best Chrismukkah Ever, The Chrismukkah That Almost Wasn't, The Chrismukkah Bar Mitz-Vahkkah, and The Chrismukk-huh?). Particulars of when exactly the holiday was celebrated were not given; Seth simply said in the first season's Chrismukkah episode that it was "eight days of presents, followed by one day of many presents," with a stress on the word "many" (this was repeated in the second season's Chrismukkah episode by Seth's new brother Ryan, with an added "many"). The only references to how it was celebrated, other than the family displaying both a Christmas tree and a Hanukkah menorah, was that the Cohens spent Christmas Day itself rather than going out for Chinese and a movie as many American Jews have taken up in recent years, watching movies like It's a Wonderful Life and Fiddler on the Roof at home while eating Chinese takeout." (Wikipedia)