I am very busy when people go out[ ]dinner.I am very busy wh

I am very busy when people go out[ ]dinner.I am very busy when people go out[ to ]dinners.
我记得课文里是I am very busy when people go out[ to ]dinners.但我在对答案是发现答案上是‘for' 题上是I am very busy when people go out[ ]dinner.一个有复数,一个没有复数.我不知道他们的区别,也不知道应该填什么.当时老师好像没注意到是否有’s'.