summer holiday/summer holidays

both are correct

"holidays" is usually talking about MANY holidays
e.g. : I will go shopping in Paris in my summer holidays!

"holiday"-just 1
e.g : I will go shopping in Paris in this summer holiday!

"Holiday" is British English and "Vacation" is American English
but they've got the same meaning and usage




Hope i can help you! ;)

我推薦您一個非常不錯的英語學習網站，里面有很多非常不錯的學習資源：
http://www.hkenglishstudy.info
希望可以幫到你!