We have built-in days for Snow Days, I think 3... But, that said, in our New England state, if there's a weather event where more then 90% of other schools, or if state offices and services close, the day is "forgiven" and we don't have to make it up.
When I was going to school, losing days and pushing back the last day of school was always a big deal... We wouldn't know when the last day of school was until April. Then in 1999, we got slammed with storm after storm and we missed so much school that there were talks of half days on Saturdays just to make sure they didn't extend school for the underclassmen until July and change the date of graduation for the seniors (as all the invites were all purchased, printed, and distributed). Another horrific storm rolled in, actually the "big ice storm" of that year, and they didn't cancel school in our district because it would have been the drop that overflowed the bucket in our threat of a prolonged school year. We were one of only a handful of schools in New England, much less NH, that didn't cancel. Unfortunately, a car driven by a student went off the road on the way to school due to the bad weather and it resulted in the death of a foreign exchange student and the surviving students were stuck in the car with a dead classmate for several hours. Word got back to the school about it, people freaked out, and they sent us all back home right after lunch, and a bus went off the road stranding a group of elementary kids for hours.
Long story short, the school system was sued to Timbuktu and back for not closing, the state was sued, and laws were changed. Now the state forgives days and schools aren't penalized, therefore the theory is that they won't keep school open when it should be closed.
But now, the last day is the last day and it's put on the calendar in Oct. It's pretty hard and fast and we'd never lose a vacation. As a result of all of that, our state is actually pretty fanatical about making sure that appropriate days are forgiven and nobody is penalized for calling off school on days where it's appropriate.