I've evovled my social planning as I got older. It consists of these two steps:

• Lower your expectations about the quantity of attendance
• Be happy and celebrate who does attend

Instead of trying to do the massively complicated action of coordinating a single event across multiple people's calendars, I do the opposite. I announce where I'll be, when, what I'm doing, and that anyone I'm sending this message to is welcome to join me. If no one shows, I am fine with that and do whatever was described in the message by myself.

Very very rarely does the entire group of recipients show. Sometimes I'll get most, occasionally a few, frequently a couple or one, and yes, sometimes none. Using this method I've found much more success maintaining friendships past 40 years old.

As we get to this age, everyone has their own lives and commitments. That's a good thing that you have friends that also have active lives.

If it's any consolation, they probably won't show up to either event.

It might not be a bad idea to say in that other event thread something like sounds fun but I have a birthday party then. Maybe you saying something will remind those people about their prior commitment. Maybe they just forgot or didn't realize the timing conflict

One thing I've seen people do is intentionally overbook. In this case, it'd be: show up at the community event for like 15 minutes, then go to the birthday party.

It probably won't make you feel any better, but those people aren't friends.