The noun season is from Old-French forms such as seson (Modern French saison), generally said to be from Latin satio(n-), act of sowing, later time of sowing, from satum, supine of the verb serere, to sow.
However, season is estación in Spanish, estació in Catalan, estação in Portuguese, and stagione in Italian, all of these words being from Latin statio(n-), whose general sense was a standing still, hence a position, a situation, a location — the sense season probably refers to the sun location in the sky according to the time of the year. Another theory is therefore that the origin of French saison is an unattested Late-Latin form satio(n-) deriving, by dissimilation, from statio(n-).
But it is more probable that the sense time of sowing of satio(n-) was extended to time favourable for specific activities, such as harvesting and hunting. Indeed, Old French had the expression il est saison, it is season, i.e. it is time (to do something). Similarly, Portuguese sazão and Spanish sazón (both from satio(n-)) have the figurative meaning of opportunity, while estação and estación refer to the calendar seasons.
Romanian uses two nouns: sezon (from French saison) and anotimp, after the German noun Jahreszeit, literally time of the year (timp and Zeit mean time, an and Jahr mean year). Similarly, Latin used tempus anni, literally time of the year, to mean season. Dutch uses the noun jaargetijde, jaar meaning year, and getijde, tide in the sense of moment in time, season (as in eventide, end of the day, evening, and Yuletide, Christmas time).
The English verb to season is from Old French saisonner, which was an agricultural verb meaning, in particular, to arrange, to prepare, to make fertile (the soil). The past participle saisonné meant, of fruits, etc., ripe, mature, and, of wood, dried and hardened by keeping, seasoned.
The French verb assaisonner, which has eventually superseded saisonner, meant to do something during the proper season, and its past participle assaisonné had the same meanings as saisonné.
From the idea of to arrange, to prepare, to make appropriate to the circumstances, stems the current meaning of French assaisonner and of English to season, which is to render a dish more palatable by the addition of some savoury ingredient.
This was, as early as the 14th century, the original sense of the English verb to season. It was only by the mid-16th century that this verb had come to mean to bring to maturity and to render fit for use by prolonged exposure to atmospheric influences, or by gradual subjection to conditions of the kind to be undergone — a specialised use of this is to season wood, to make wood suitable for use as timber by adjusting its moisture content to that of the environment in which it will be used.
Hence the figurative sense of seasoned, which is accustomed to particular conditions, experienced. It is comparable to that of salted, which literally means preserved with salt, and figuratively, of a horse, having developed a resistance to disease by surviving it.
We usually understand seasoning as meaning a heightening of flavour, but it also had the sense of moderating; in The most excellent Historie of the Merchant of Venice (between 1596 and 1599), by the English poet and playwright William Shakespeare (1564-1616), Portia says to Shylock:
(Quarto 1, 1600)
The qualitie of mercie is not straind,
it droppeth as the gentle raine from heauen
vpon the place beneath: it is twise blest,
it blesseth him that giues, and him that takes,
tis mightiest in the mightiest, it becomes
the throned Monarch better then his crowne.
His scepter showes the force of temporall power,
the attribut to awe and maiestie,
wherein doth sit the dread and feare of Kings:
but mercie is aboue this sceptred sway,
it is enthroned in the harts of Kings,
it is an attribut to God himselfe;
and earthly power doth then show likest gods
when mercie seasons iustice.
“The qualitie of mercie is not straind, it droppeth as the gentle raine from heauen” alludes to the Book of Ecclesiasticus, 35:20, in which seasonable appears:
(King James Bible – 1611)
Mercy is seasonable in the time of affliction, as cloudes of raine in the time of drought.