Would "jesién jest porą roku" also be accepted?
Are "pora roku" and "okres" completely the same thing?
no. "Pora roku" = "season of the year", "okres" = "period".
'the fall' not 'fall' I think. Fall is not the name of the season.
Autumn is the official name of that season in English. However, Fall is used as well. "The leaves fall in Fall." Spring, Summer, Fall and Winter would be acceptable names of the seasons.
Fall is American English, autumn is British English.
Why not "Jesień to/jest roku?"
It means something like "Autumn is of year". It doesn't make much sense, neither semantically nor grammatically.
The Polish phrasing for "season" (meaning: Spring/Summer/Autumn/Winter) is "pora roku", which could be translated as "time of the year".
Why not the season of the year is accepted?
"a season of the year" is accepted.
"the" doesn't make sense here, I believe. Usually we speak about four seasons, so autumn is only one of them, "a season".