秋天 秋季 http://www.italki.com/question/177315?answer-sorting=3 一年有365天 一年有4季 約121個夏天 組成1個夏季 若不予以嚴格區分 二詞可通用 但是 某些情況 必用 "夏季" 如 夏季季風 夏季作物 因為 季風不是吹拂 幾天而已 農作物也不是三天兩天就可以收成 又如 夏季是台灣的豐水期 因為時常有颱風

夏季：Summertime 夏天：Summer day

夏季 is about season and 夏天 is about a single day but also about season.

They are almost the same. If you have to tell that small difference beween the 2 expression, I think 夏季 has more clear fuction to discribe a "season“, more literary and formal. 夏天 more informal and oral. When we write an artical, we can use both; when we speak, we often use 夏天.

but I also have to say 夏季 is a formal style. please use 夏天