The mathematics of state-required instructional days
Most US states require somewhere between 175 and 185 instructional days per school year. Districts publish a calendar that hits the requirement plus a small buffer — usually three to five "snow day" days. When weather closures exhaust the buffer, the district must add make-up days, which usually come from the spring break, the end of the year, or a Saturday school session.
Reading the buffer on a printable calendar
The PrintCalendars academic-year overview shows the published first day of school and the published commencement date. The number of instructional days between the two — minus the federal holidays, minus the published breaks — is the actual instructional total for the year. If the published total is more than your state's requirement by three to five days, that's your weather buffer. If it's exactly at the requirement, the district has no buffer and the first weather closure will require a make-up.
Where make-up days usually come from
The most common make-up day choices are: extending the last day of school by one day per snow day used, converting one of the published spring-break days into an instructional day, converting Presidents' Day into an instructional day, or scheduling a Saturday school session. Districts vary on which choice they make first; the published calendar's footnotes usually spell out the policy.
Why this matters for families
If your family books summer travel based on the published last day of school, you are gambling that no make-up days will be added. A safer approach is to print the academic-year overview, count the buffer days, and add a buffer of your own to summer travel plans. The PrintCalendars overview makes this count visually obvious — the federal holidays are pre-shaded, so you can count the unshaded weekdays directly.
The mid-winter check
The right time to check the snow-day buffer is the first week of February. By then any weather closures from the fall and December are baked in, but the spring is still flexible. If the buffer is exhausted, expect changes to spring break or to the last day of school; if the buffer is intact, the published calendar will hold.