The rule is simple, if I have nothing to do at that time, then I go to the event.
The past year I have been missing a lot of social activities. I had been really meaning to go to them, but for one reason or another I simply do not go. I reached a point of complete self-hatred when I missed an event I was looking forward to, as it gave me a chance to meet completely new like minded people, but my sleep schedule was bad and I overslept. That threw my whole day off. As I went for a walk I realised that I just needed to be 10 times more social, even though I am quite introverted by nature.
To clarify the new rule, I will always be busy during the usual work hours, which are weekdays until 5pm. After that I will be free to socialise. If I find an event I am even vaguely interested in I will go to it. If I plan on going to the gym I will do so.