When Jewish people celebrate Christmas for various reasons i.e. they like the feeling of the trees, presents, etc; they feel left out; or just because they want to celebrate Christmas in a nonreligious manner.
Sydney: Yeah, Jamie, I got Uggs for Jewmas!
Jamie: No way! I got a mac laptop!
Sydney: Whoa! I love Jewmas!
When a Jewish family or a single person of the Jewish faith celebrates Christmas, which is a Catholic-based holiday. They give and receive gifts and often times include Santa Claus (commonly referred to as "Saint Nick"). They also often do not respect the Jewish holidays occurring around the time, as these holidays bring less gifts and food as Christmas.
Dude, Josh is celebrating Jewmas again. What a Jew.