Which of the following is correct when addressing an audience or more than one person? Or are both equally valid?
I hope you have all had wonderful summers!
OR
I hope you have all had a wonderful summer!
Which of the following is correct when addressing an audience or more than one person? Or are both equally valid?
I hope you have all had wonderful summers!
OR
I hope you have all had a wonderful summer!
Replace summer with another word to see how it fits:
I hope you have all had wonderful dinners
Sounds clumsy - you're implying that each individual has had more than one dinner.
I hope you have all had wonderful times
Even clumsier.
So:
I hope you have all had a wonderful summer
I hope you have all had a wonderful dinner
I hope you have all had a wonderful time
So, while many of them have had the same experience, each person has only experienced the one thing
I hope you have all had wonderful summers!
OR
I hope each of you had a wonderful summer!
In the first sentence, we have a plural audience (you have all). We can also assume that each person had their own form of summer vacation, independent of the others. Given that, the object should be plural, i.e., summers, not summer.
To explain further: If members of the plural audience had been served the same meal or gone on the same day trip, and then reconvened, one would say:
I hope you have all had a wonderful meal OR I hope you have all had a wonderful day trip. [singular objects]
On the other hand, if they had all gone their own separate ways for meals or day trips, and then reconvened, one would say:
I hope you have all had wonderful meals OR I hope you have all had wonderful day trips. [plural objects]
I have consciously refrained from rewriting the OP's example.