I'm going to explain this horribly. In the US, each state may be slightly different but school committees are required by law to schedule a certain number of days of instruction. In my state it's 185. Schools are required to physically be open a certain number of days as well. In my state it's 180 days. So there are five days that can be taken off depending on any emergency, and usually that's weather related.
In order to not have the official school calendar change its ending date (or taking away holiday vacation days) every time a snow day has to be taken, the school committee may build in snow days into the calendar. How it's typically done is, there are a certain number of built in days allotted between, say, December and April 1st 1st and as you reach each milestone if in the calendar, if days off haven't been needed for emergency, the school closes on the built-in day. This ensures the students get the 185 days of instruction and the school year ends on time. If a snow day WAS needed, the school is open on the scheduled makeup day.
So it would have been known to anyone who has access to the official school calendar (every school provides a link on their official website) well in advance that Monday Feb. 13th was a potential makeup day. If you were reasonably paying attention and knew it was a mild winter without a lot of school closures announced, you could reasonably assume that a makeup day might be taken that day as scheduled, having not been needed earlier.