After 1976, they didn't really have "seasons", since they only had about 3 or so new weeks per year, and aired mostly reruns. The times of year that the new episodes aired changed a lot over time:
1979-1983: the new weeks seemed to air at random times
1984-1989: 3 new weeks a year, in November, winter, and May. The November week was always during Thanksgiving week. The winter week was in either February or March. The May week was usually the first week of May (except in 1985, it was a week later, for some reason)
1990: There was no new week in the winter; instead, the Environment week had its first airing in April 1990. Instead of a new week in May, they had one in July/August. The November week was Thanksgiving week, as usual
1991-1992: There was still a November week during Thanskgiving week, and a winter week (now always in February), but the other new week is now in August (rather than May). The August week is always the last week of August.
1993-1994: There are now only 2 new weeks: one in February and one in August. Starting in 1993, until 2000 (inclusive), the February week is during the 3rd week of February (the week of Presidents Day). The August week is still the last week of August. Since there was no longer a new week for Thanksgiving week, Food started airing that week.
1995: In addition to the usual February and August weeks, there was an additional new week, Mad Feelings, in October.
1996-2000: In addition to the usual February and August weeks, there was also a new week in July, in 1996, 1998, and 1999. There was no new week in July 1997 or 2000 (I think one was planned in 1997 but was scrapped, probably due to Johny Costa's death. I don't know if one was ever planned for July 2000)
2001: Only one new week, during the alst week of August. Not sure if one was ever planned for February (or even July).
The published schedules were always from Sept. 1 to Aug. 31, roughly Labor Day to Labor Day. The only months to never have a new episode are January and December. There was never a new full week in September, but some of the late August weeks continued into September.
I wonder why they had so many new weeks during Thanksgiving week. I would have thought that fewer people would have watched that week than usual.