在夏天是用in summer 还是on summer

在夏天是用in summer 还是on summer 是in summer还是on summer 在暑假是on summer还是in summer还是at summer 请问是in summer vacation还是on summer vacation? 是on summer holidays还是in summer holidays? 是at the summer 还是 in the summer 还是on the summer 在夏天早晨时 on the summer morning 还是 in thesummer morning/? 是in summer 还是in the summer 在my summer vacation前面有on还是in. 是in summer holiday还是on summer holiday有没有规律?总结一下! at summer 还是IN SUMMER a hot summer day 前面用in还是on