I showed up at the house of a friend and she was clearly surprised to see me. We still had a visit-- coffee and food I brought. She seemed a little flustered at first and when I asked if everything was okay, she said, "Well I thought you weren't coming because we never confirmed".
I said, "Oh right but you invited me a while back and I said yes great, so..."
Her: "Right but that was a while back and we never confirmed".
We finally dropped it and had a nice visit but was left wondering whether I'd missed some convention.
YES: an invitation and acceptance made over a month in advance needs to be refreshed with a confirmation. Otherwise it expires.
NO: If some issues a specific invitation and you accept, it's reasonable to just stick the event in your diary and rely upon it.