A day in winter when a large amount of snow falls in a short amount of time making travel difficult. Schools close for this reason. Usually enjoyed unless you are older in which case you have to shovel. Has become increasingly common in the Northern Midwest and New England over the past few years.
Guy 1: Hey did you see the weather forecast?
Guy 2: Yea, 15 inches of snow tomorrow.
Guy 1: You know what that means...
Guy 2: SNOWDAY!!!
Teacher: Hmm chance of snow for next week.
Student: it will be a snow day.
Teacher: No it wont
Student: Yes it will we live in Wisconsin.