The classes won't get listed til about February or March. Until then they don't know if they have the staff.
A lot of courses, though, are offered every single spring and summer. All first year econs and 2g03/2GG3/2H03/2HH3, for example, as well as all first and second year core math (except STATS 1D03 which is only ever offered in one session in winter).
A lot of other first year intros, like physics and chem, are always offered every summer.
says thanks to old guy for this post.