I know of school boards that do Tues/Thurs for JK and Mon/Wed/Fri for SK. This makes sense to me as they only ever go on their set days every week and if there happens to be a holiday they just miss that day. This at least allows for easy daycare planning as the daycares offer a Jk spot and an SK spot to make up a full spot.

That said, I suppose like anything you just adapt to what you know so it may very well not be as complicated as it seems!