Schools around where I lived when I grew up did this: A certain amount of snow days were budgeted in the schedule. Once spring hit if we had extra snow days we

Schools around where I lived when I grew up did this: A certain amount of snow days were budgeted in the schedule. Once spring hit if we had extra snow days we didn't use for snow they would use them up so we'd just have a random couple of days off in the spring. It's likely they felt comfortable using a "snow day" if they had a few extra.