I didn't mean to say that a picnic was wrong. I personally think picnics is better, though some may prefer otherwise.
I was trying to give you in post #5 what I think is the simplest and clearest rule of use.
Thank you for the kind reply,
jutfrank. 
I just found more information about this. I hope it also helps.
Practical English Usage 3rd
530 singular and plural (8): distributive plural
1 people doing the same thing
To talk about several people each doing the same thing, English usually prefers a plural noun for the repeated idea.
Tell the kids to bring raincoats to school tomorrow. (More natural than Tell the kids to bring a raincoat ... )
Plural forms are almost always used in this case if there are possessives.
Tell the children to blow their noses. (NOT ... to blow their nose.)
Six people lost their lives in the accident.
Uncountable nouns cannot of course be used in the plural.
They were all anxious to increase their knowledge. (NOT ... their knowledges.)
2 repeated events
In descriptions of repeated single events, singular and plural nouns are both possible. When no details are given, plural nouns are more natural.
I often get headaches. (NOT I often get a headache.)
She sometimes goes for rides over the hills.
When details of the time or situation are given, nouns are often singular.
I often get a headache when I've been working on the computer.
She often goes for a ride over the hills before supper.
Singular nouns may also be used to avoid misunderstanding.
I sometimes throw a stone into the river and wish for good luck. (NOT I sometimes throw stones ... - only one stone is thrown each time.)
To refer to the time of repeated events, both singular and plural expressions are often possible with little difference of meaning.
We usually go and see my mother on Saturday(s).
He's not at his best in the morning(s).
3 generalisations and rules
In generalisations and rules, singular and plural nouns are both possible.
We use a past participle in a perfect verb form. (OR We use past participles in perfect verb forms.)
All documents must be accompanied by a translation of the original. (OR All documents must be accompanied by translations of the originals.)
Mixtures of singular and plural are possible.
Subjects agree with their verb.
Children may resemble both their father and their mother in different ways.
This often happens with fixed singular expressions like
at the beginning.
Discourse markers usually come at the beginning of sentences.