Best definition
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!