Summer ought to be OK. But I can tell you from experience not to bother on Easter weekend.
I had an opportunity to go two Mays ago.
I loved it, the history is unbelievable.
Unfortunately very crowed, and there is no way to linger and look at the art, as everyone is sort of rushed along. You can linger a bit in the Chapel, but those guards will get you if you speak above a whisper.
We went to Peter’s Church, just as beautiful and only a handful of people.