In large swaths of the US, schools (especially grade school and high school) typically close for some number of days each year due to snow. There can be other weather events too but snow's the common one.
Most schools in the midwest US actually build a school calendar that is 7-8 days (or so) longer than is actually necessary. That way if they have to take snow days, they're not going until July; alternatively, if they don't take snow days, everyone gets excited to be let out early. . . when it's really the legal time to be out of school.
I've never heard of a school actually letting out early though. Last year my son had to go an extra week though because they used more than the planed days off.
