When the weather contradicts what the season should dictate — for example, by delivering Summer-like warmth on an April day (unexpected in either hemisphere) — we say the weather is “unseasonably warm”.
When savoury food isn’t salt-and-peppery enough, we say it’s unseasoned (and probably unsavoury).
So why “unseasonably warm”?
Hint: it’s not because we can’t improve the weather with salt and pepper.
The OED lists season (n.) as “any one of the periods, longer or shorter, into which the year is naturally divided by the earth’s changing position in regard to the sun” (and so on). That’s all based on an early version of the verb to sow (thus “sowing time” , from Latin and Vulgar Latin via Old French and Middle English, with variations in Modern French, Catalan, Portuguese, Italian). Humans have been talking about seasons since about 1340.
The concept of seasoning (v.) — “to render more palatable by the addition of some savoury ingredient” — turned up in around 1400, and is based on the same Old French concept: we leave fruit to ripen fully with the seasons, becoming as delicious as possible before we devour it or bake it into a pie.*
So unseasonal weather makes sense to me. But why unseasonable and unseasonably? Unable to be seasoned?
The construction is literally just season + the able suffix.
Season+able (adj.) means “occuring at the right time or season”.*
Season+ably (adv.) means “in a fitting time; at the right moment; in due season”.
But if you ask me, the real answer comes back to a noun: seasonableness.
So if something is unseasonable it’s unsuitable or unreasonable.
When I comfortably wear a sundress in April — or am forced to wear a woollen scarf in December** — I do so because of weather that is unreasonably warm or cold (within the context of the season). I am unable to provide a reason for such inexplicable weather …Except to say that we should consume less, recycle more, and look for alternative sources of low-impact, renewable, sustainable energy.
And all this still has very little to do with salt and pepper, other than the fact that a cold day in Summer is generally thought to be unsavoury.
* As opposed to seasonal (adj.), which means “depending on or happening with the seasons”. In Japan, cherry blossoms appear seasonally; if they flowered all through the year they would be unseasonal — unrelated to or unaffected by the prevailing season. Which means you could talk about an “unseasonally warm day” and still technically be correct, but you lose the element of unreasonableness and incongruity.
** That one is hemisphere-specific.