Panfilo
Existential Risk
A lot of reasonable stuff is being said about quorum and regularity, so I'll take a different approach. Part of solving this problem, if it comes up often (though optimally before), is finding out explicitly just where your game night falls in individual people's life priorities. I've known one person who considered D&D about as important as a hot date with a long-time crush, and another who bailed on a session to get a normal, non-event-related haircut. It'll depend on how well the campaign is going, as well as personality types IRL. Get an idea of where you stand compared to their other obligations (and what those are), and you won't be blindsided nearly as often.