There was also 2 official 16-month calendars that were released. I believe for 1988-1989 and for 1989-1990. Each month contains a Calvin and Hobbes strip.
The second one was reusable for 2006-07 and 2017-18, and will be reusable again in 2034-35, 2045-46, 2062-63, 2073-74, and 2090-91.
The second one is reusable twice as often as the first--not something I would have guessed. (I'm still not sure I believe it...).