Hello !

I've been DMing for a few years now, but most of the campaigns tend to be sandbox-y type of games. In the past, games have usually just gone on until other commitments got in the way, and the current one I'm running is on session 24 with easily another 25/30 sessions until the plot wraps up. 

All that being said, a couple of friends want me to run a campaign but need to be done by the middle of August, totally about 8 weekly sessions. Since most of them are relatively new, I figured a 5-6 session campaign, roughly 3-4 hours a session, would extend to 8 weeks once you add in time of helping them figure out mechanics. Four of the friends will be able to continue after August so I was thinking of continuing it with their characters after the other 2 leave, but I didn't want to steal a narratively satisfying ending from the 2 that are leaving. 

So, how do you guys plan for shorter campaigns that have a full/complete plot. 

Any tips? 

I think doing it module style with one session locations would be a good idea (Generally each location in the modules I have seen have each location be a session's worth of content). Plop down a tutorial level location (first session) which the PCs are immediately thrust into the action. Then either have a linear set of locations or adventures after that, or have more of an open world (maybe a choice of 3 different adventures where the players can choose which order to tackle them in, which increase in monster power rating as the other quests are done), and then a final climax.

Since I usually use published adventures, my advice will be focused in that direction.  Here are a couple of options:

1) Run Lost Mine of Phandalinm(LMOP); it's an excellent starter adventure, and I think it would fit in your time frame.  You might want to opt for milestone leveling instead of XP, so that it's a bit easier to trim options if it looks like you are running out of time.  It's a nice combination of an overarching plot, and sidequests that provide a sandbox feel.  Given that you are an experienced DM, you might also pick up Dragon of Icespire Peak and work some of those sidequests into the plot.  If you do some googling, you can find some guides to combining the two.  Even if you don't use the pregenerated characters, I'd encourage you to take a look at the backstories, as they provide some nice hooks into the sidequests--and in at least one case part of the main quest.    LMOP has a clear ending with a relatively low-level boss, and would provide a good "breakpoint" so that the players that are leaving could bow out at that point in a way that makes sense narratively and the rest could continue if they want.  You could homebrew things after that, or make use of some of the "after Phandalin" adventures on DMs Guild, or segue into another published adventure set in the Sword Coast.

2) Use a series of one shots are more or less loosely connected; that way each session or two reaches a "conclusion" where it makes sense for a character to bow out, but the story continues.  It's not a level 1 adventure trilogy, but the Fey's Bargain series is one example of what I mean.  AL modules would be another possibility: Pick a season and run the modules in order as you have time.  AL modules are available on DMs Guild.  The free AL Player & DMs Pack contains a content catalog that lists all AL modules by season with information about run time, recommended levels, etc.

I would also recommend LMOP. Ghosts of Saltmarsh is also a good resource for adventure.