means Happening, available, or characteristic of a particular time of year, rather than lasting all year round.

from From 'season,' which came through Old French 'seison' from Latin 'satio' — literally 'a sowing,' the act of planting seed. So a 'season' was originally the right time to put seeds in the ground, and 'seasonal' still carries that ancient sense of the calendar dividing the year into windows when certain things can and cannot happen. (The cooking sense of 'season,' as in salt and pepper, shares the same root by a different path — to 'season' food once meant to ripen or mature it.)