《朗文》：

on holiday 

I’m away on holiday until the 1st of June.

in the holidays 

He came to stay with us in the school holidays.

GRAMMAR: Holidays

You use holidays when talking about a period when you are not working or studying, or when you are travelling.

• You say the holidays:

Soon it will be the holidays.

• You say my/your/her etc holidays:

Where do you want to go for your holidays?

holiday既可以是不可数名词，也可以是可数名词。on holiday中为不可数名词。in the holidays中为可数名词，且要用复数，必须用特指限定词如the, my, our等。

所以网友的句子中in holiday为错误，必须改为on holiday或者in the/my holidays.