I can tell you from experience that the last time WBWC went dark over a holiday or summer break was no later then 1974. Prior to the mid 1970s, these mostly 10-watt stations viewed themselves as just another campus club or activity focused almost solely on the campus even though the signal covered the adjoining communities. In WBWC's case, the student operators began to look at the FM station as a service to the community, and the loss of listenership by going dark became a serious issue. However, the broadcast days were shortened as the few truly dedicated students and volunteers did their best to keep the station on-air.
It appears that prior to that, the only time during a break that the station remained on was during the summer of 1969. In a 50th anniversary interview with the then-student General Manager, Jerry Baron, the station found out that they had shown up in the ratings, and this exciting news lead to a year of a concentrated effort to remain on-air and provide a better quality of service.