Describes something that occurs, changes, or is typical of a particular season. Often used with nouns like 変化 (change), 商品 (product), 仕事 (work), or 傾向 (trend).
This work is seasonal, so it's busy in winter.
Please be careful of seasonal temperature changes.
Formed from 季節 (season) + 的 (-like, -al), a common pattern for creating adjectival nouns.