Seasonal and seasonable are sometimes interchanged, probably because of their obvious connection with season. In edited prose and in formal speech these two adjectives are almost always distinguished. Seasonal describes phenomena that occur with or depend upon a season or the seasons: seasonal fluctuations in rainfall; seasonal sales. Seasonable in reference to weather means “suitable to or characteristic of the season”: a seasonable December; seasonable temperatures for July. Seasonable also has the sense “timely, opportune”: a seasonable offer of financial assistance.

Did you mean unseasonal?