Hi Heidi. You could actually say both. Sometimes we say, "I went to summer camp last July." and it has the same meaning as "I went to a summer camp last July".

Only the second sentence is OK. I went to a summer camp last July.

They are both ok but they have a different emphasis. I went to summer camp last July. It was really good for me because I made many friends there. (People know what "going to summer camp" is) I went to a summer camp last July. This camp was over 50 years old. (The focus is on the camp)