The rules don't specify any distinction between season downtime or weekly downtime. Only that you must broadcast "12 hours per day each day of the year".
(b) All stations, including those meeting the requirements of paragraph (a) of this section, but which do not operate 12 hours per day each day of the year, will be required to share use of the frequency upon the grant of an appropriate application proposing such share time arrangement.
The last license renewal go-around when Marty Hensley of "Hoosier Public Radio" (actually a religious outfit, not NPR) filed a bunch of 73.561(b) challenges, some were IIRC against stations that were on the air for more than 12 hours a day, every day, during the academic year...but they were not making the 12 hour minimum over summer and winter breaks. As I heard it, in most cases the only reason his challenges were dismissed was because he didn't formally propose a share-time arrangement to the school, get rejected, and THEN file with the FCC to force it. He skipped straight to the "file with the FCC" part and that's not how the rule works; probably figured that if he did that he'd tip them off and incur negative publicity. Which is probably true, but it's also irrelevant. The rule doesn't give any indication as to how much downtime allows a 73.561(b) challenge. In theory, just ONE DAY when you don't make the 12 hour minimum, during the ENTIRE EIGHT YEAR LICENSE PERIOD, is enough to leave you vulnerable. Although I would think anyone who filed a challenge with the FCC based on one day's downtime would, at a minimum, have to prove it.
Mind you, this is all at the ultimate discretion of the FCC. I know that the bulk of FCC employees don't want to see stations forced into 73.561(b) share-time challenges if it can be avoided, no matter how insignificant the technicality necessary. And the FCC does have a history of conveniently ignoring its own rules on the odd occasion that it wants to. Or when Congress wants it to.