Any arbitrarily taken holiday or unoficial vacation. Usually celebrated in schools but can be celebrated during any other event. The day consists of taking the day off from school, work, etc and just having fun.
Steven-Hey are you coming to school tomorrow?
Beanerous Brokus I – No, Im cellebrating yadnuf tomorrow
Steven – ooh i didnt know you were jewish
Beanerous Brokus I – Im not you muckfluck!