The day has come. The deadliest monster in all of Dungeons and Dragons. The one monster nearly impossible to defeat. A monster known to wipe out entire parties and destroy worlds. A ravenous beast so horrific and so terrible it takes intricate preparation and planning to have a chance of success. The one monster more feared than the mighty Tarrasque or even the Gods themselves. Scheduling!
Let’s face it. We all have lives. We have things to do outside of a TTRPG. And games like DnD take a lot of time. Most campaigns will take years to complete, and each session can be an all-day affair. That’s a lot of time to invest. Especially if you have school, work, a spouse, kids, or any hobby or interest. Multiply that by five players plus the DM and it can seem like a miracle any campaigns get off the ground at all.
It’s inevitable something will come up and someone will have to miss a session. The question is, what do you do about it? Well, the first thing is everyone needs to be understanding about it. It’s a little hard to say to your friend that they should prioritize the game overseeing a child’s school play. TTRPGs are a fun hobby but don’t let them interfere with real life. If something comes up, don’t quit your job so you don’t miss out on the Tomb of Horrors.
Session zero is so important for many reasons. During a session zero, the guide rails of the game and the table are established. The GM introduces the world and players build their character’s backstory and get a sense of how everyone fits into the world. One of the things to discuss is what happens when someone is absent. Because it’s going to happen. 
First thing is to ask how long they are going to be absent. If they are just going to miss one game, it’s not that big a deal and there are options on how to progress. But what if they will miss several games? Every situation is different so it’s hard to give a concrete rule on how to deal with it because every gaming table is different and every group is different. If there are five players and one has to miss for a few weeks, it’s less crippling than if that same person misses a few weeks but there are only three players.
If someone is going to be absent for an extended period of time, make it part of the story. This is especially true if that player gives notice to the group that they will be absent for a few weeks. Give that character something to do outside of the main story. Maybe they have to smuggle someone out of town. Maybe they need to stay behind and coordinate the city cleanup efforts. Maybe they just need some time alone to think and hone their abilities. It’s entirely up to the DM and the player. Make it epic. Make it grand. Make it memorable. That way, when the player comes back, they might have some new knowledge to share with the group. And there’s your hook to not only integrate them back into the story but move the group to the next adventure.
Another option is the classic kidnapping scenario. It stalls out the main story a bit but it gives all the other players something to do while one of them is away. Find them, rescue them, and have some fun with the aftermath.
Perhaps it’s something that came up unexpectedly. Well, the DM can always use that person’s character as an NPC. This can be a bit of a problem because the DM knows the story. All of a sudden that character can’t really be part of the story unfolding. When combat eventually happens, the Dungeon Master knows the battle tactics of the monsters. The DM would know the secret weakness the players need to survive. It’s up to the DM how active they want to be with that person’s character. One thing I’ve done is leave it up to the group what that character does. The DM can ask, “what does this person do?” Attack, run, search for traps, etc.
The one option I don’t think anyone is happy with is everyone just decides not to play until everyone can. Suspending the game is an option. If several people are going to be absent, it’s usually the best call. If you’re at a point in the story where everyone really wants to be there, then it’s cool to reschedule. It’s disappointing to think that someone must miss out on the climax of the story because of unforeseen circumstances.
