"Don't bother with the one behind the mirror on the fourth floor. We used it until last winter, but it's caved in -- completely blocked."
-- Fred Weasley (PA10)
A very roomy secret passage to Hogsmeade was located behind a big mirror on the fourth floor, but it caved in during the winter of 1992-1993 (PA10).
The term "last winter" could mean anytime between the end of November 1992 through February 1993, but in order to fit it on the timeline we are assigning it a date of January 1993. The season and year are canon, the exact month is just a reasonable guess.