Answer:
The subordinate clause is: "when we went to the Fourth of July picnic."
Step-by-step explanation:
A subordinate, or dependent clause, does not express a full idea on its own. It needs the information expressed by the main clause in order to make sense. Thus, if we only said or wrote "when we went to the Fourth of July picnic", the listener or reader would be curious to know what happened.
Besides that, a subordinate clause is formed by a subject and a verb, and it starts with a relative pronoun or a subordinate conjunction (in this case, "when").