We all have those times when 1 or 2 players cancel out on game day. Everyone else can come, but you can't play w/o the others. Should you cancel the session? NO! Try playing a one-shot game instead.
There are numerous one-shot rpg games available and many of them are free. Some of them take little to no time to learn and are a lot of fun. Below is a link to a list of short reviews that I do on these games. So check out this growing list and plan ahead for your next cancellation.