seasonal (adj.)

c. 1300, sesoun, seson, "a period of the year," with reference to weather or work, also "proper time, suitable occasion," from Old French seison, seson, saison "season, date; right moment, appropriate time" (Modern French saison) "a sowing, planting," from Latin sationem (nominative satio) "a sowing, planting," noun of action from past-participle stem of serere "to sow" (from PIE root *sē- "to sow").

The sense shifted in Vulgar Latin from "act of sowing" to "time of sowing," especially "spring," regarded as the chief sowing season. In Old Provençal and Old French (and thus in English), this was extended to "any one of the four natural periods of the year," especially as determined astronomically by solstices and equinoxes. Later it was extended to the recurring annual wet and dry periods of the Tropics (1719).

In other Indo-European languages, generic "season" (of the year) words typically are from words for "time," sometimes with a word for "year" (as in Latin tempus (anni), German Jahreszeit). Spanish estacion, Italian stagione are unrelated, being from Latin statio "station."  

The season, short for some particular annual festivity, is by 1791 (hence season's greetings, etc.). Sometimes merely meaning "period of time," as in for a season. Man for all seasons, one for all times and circumstances, is from 1510s.

The meaning "time of year when an animal is hunted or killed for food" (as in in season) is from late 14c. The sense of "period of time regularly devoted to a particular sport or amusement" is by 1680s. Meaning "time of year during which a place is most frequented" is from 1705. Season ticket, one giving the holder unlimited use, admission, etc. for a specified period, is attested from 1820.

suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).