Heat and humidity are seasonable here in summer means “they’re normal for this season of the year.” Sentimentality is seasonal at Christmas means “it’s typical or characteristic of Christmas seasons.” Seasonable can also mean “opportune” or “in time,” as in Their arrival was seasonable, just when we hoped for it. If people arrive seasonably, they are on time or possibly even a bit early; if they arrive seasonally, they visit annually at about the same season of the year. Never use seasonable for seasonal (the other possible confusion almost never occurs). Unseasonable, unseasonal, unseasonably, and unseasonally are precise antonyms of seasonable, seasonal, seasonably, and seasonally respectively.
