noun, noun which may take the genitive case particle 'no'
Refers to the time when a particular food (fruit, fish, vegetable) is at its best and most plentiful.
今はイチゴが旬です。
Strawberries are in season now.
旬の魚はやっぱり美味しい。
Fish in season is really delicious after all.
noun which may take the genitive case particle 'no', na-adjective
Extended use for things that are currently popular or in fashion, often with a nuance of being 'hot' or 'fresh'.
彼は今、旬の俳優だ。
He is a popular actor right now.
このデザインはとても旬な感じがする。
This design feels very trendy.
季節 is a general word for 'season' (spring, summer, etc.), while 旬 specifically refers to the peak season for foods or trends.
流行 means 'fashion' or 'trend' as a general phenomenon, while 旬 emphasizes the peak moment of popularity.
The kanji 旬 originally referred to a ten-day period, but in modern Japanese it has come to mean the peak season for foods and, by extension, the height of popularity.