It really depends on the part of the country - and what you need the information for.
If you're looking at a location that mostly draws a local crowd, you would want to know when those districts are likely to take their breaks. If you are looking at a place like Orlando that draws crowds from around the country, you would need to consider both the local crowds and the general pattern around the country.
For Orlando, the peak school break time would be the weeks before and after Easter. But many school systems in New England base their breaks on Presidents' Day (Feb) and Patriot's Day (NOT Sept. 11, but the original Patriot's Day which falls on April 19, and commemorated the battles at Concord and Lexington (and Paul Revere's famous ride).
Also consider that some states vary more than others - Dave has 2 different dates listed for CT, yet our District and several surrounding don't take either week but are evenly split between 2 other weeks.
Then of course, as for fixed week#, for those schools that schedule spring break around Easter, that had about a 6-week window that jumps around, so no fixed week will cover it. For the others, like Massacusetts, a single fixed week will cover you about 75-85% of the time, as the calendar shifts.
Consider that Thanksgiving is usually week 47, and Christmas usually week 51, but not always - and which years differ also depend on the individual resort.