Prepared my first own Pathfinder Campaign over the last week. Three hours before it starts someone calls it off.
A week of work thrown in the garbage. Feels good.

DM Lesson #1: always expect people to bail at the last minute, and prepare for that in advance.
Yes, it's annoying, but unfortunately that's the reality of it.

While there can certain events that require the presence of the whole party, it's good to have some wiggle room to adapt and improvise.

Sometimes life just gets in the way. Other times, some people are just not very committed to the game. In the latter case, you may want to consider getting rid of them asap.

@endor We were already low on people, this was just the last straw. He is usually one of the most committed to the game. I don't know why he bailed.

@Marl3x in that case, I'd guess that he had something unexpected/urgent to take care of. Life happens, you can always postpone.

@Marl3x scheduling games is one of the hardest things to do, this kind of stuff just happens. It's happened to me so many times for my own campaign too.

I would suggest that you take this as an opportunity to add even more stuff to your campaign (never enough lore amirite). Add more NPC backstories, interesting puzzles, practice voice acting that eldritch space monster, add a few more t w i s t s to that plot, whatever.

@RememberMe Exactly what I am going to do. I'm playing a one-shot and I find some parts of the adventure to be really strange or just dumb, so more time for polish is welcome.