IPA: //ʌnˈsiːzənəbli//
KK: /ʌnˈsiːzənəbli/
In a way that is not typical for the current season or time of year.
The weather was unseasonably warm for October, making it feel like summer again.
Unseasonably → It is formed from "un-" (meaning not) and "season" (from Old French *saison*, meaning a time or period). The suffix "-ably" (meaning in a manner) indicates how something is done. The word "unseasonably" means not in accordance with the usual time or period, often referring to weather that is out of character for a particular season.
Think of 'not' ('un-') being combined with a 'time' or 'period' ('season') to remember that 'unseasonably' describes something that is not typical for that time.