It's often easier to explain things verbally than in writing. For example, you could say to your friends (right now) something like this:
"Since I'll be twiddley-umpty-five on the umpteenth of November, I'm planning on inviting a few friends round on that evening for a few drinks and snacks. It won't be a proper 'party' but it would help to make my day if you could come along. So please keep that date free in your diary, if possible"
Then later, if you stil want to put something in writing, you can send out cards (or emails) headed "A Reminder" and adding something like "I'll be as old as the hills on the umpteenth of November and I'm hoping that you've not forgotten that I'll be opening a few bottles, and providing a few snacks, at my place that evening (from 7.00pm until about 9-ish). I do hope that you'll be able to join me".