I suspect it is surprising to outsiders that theater people end up with things scheduled right on top of each other, resulting in having to juggle substitutes, re-work rehearsals, and of course do lots of terrible crunches -- ten hour days in tech on one show and going home to work on drawings for another that has to go to the shop by Friday.
There's two reasons it happens more often than you would hope.
First is that seasons overlap. The summer stock season starts building before the regular season is over, and is still in performance when the first shows of the next regular season are already in rehearsal. Theaters also overlap; if there are two theaters sharing an audience they often try to stagger their shows so one opens on one weekend, the other opens two weekends later. And individual theaters also overlap, even those that only have one stage to work on; since rehearsals and build go back five or six weeks, and run is five or six weeks, you will be in the middle of prepping one show long before the previous one closes.
The second is that schedules change. Opening night is a given -- perhaps the only given. But performances change; shows get extended, the "optional" Thursdays get added or cut, extra shows are crammed in. And sometimes a big group calls and a special performance is set up for them on a Wednesday or something...something that didn't come close to appearing in the original schedule.
And practically all of the key days fluctuate. When you are talking a musical, and you are in Sound, then there's first orchestra rehearsal, sitzprobe, first dress, the first full dress with orchestra...and all of these dates remain in the great unknown until access to the venue, progress in rehearsal, musician's schedules, the availability of the leads, etc., etc., etc., gets hammered out.
Which means, in short, you sign a contract saying, "I will be there on that day" many months before anyone knows what day that may be. Do that for two different productions...and you've got a problem.
Sunday I worked two performances plus load-in and load-out and a long drive in the rain back from the venue de jour. Then across town to watch as much rehearsal (on another show) as I could keep my eyes open for.
Today called in at my regular work and spent twelve hours hanging lights then watching rehearsal, without, of course, a meal break. At seven tomorrow morning I head out to another load-in and performance of Sunday's show, then probably back across town to try to get in as many hours of hanging lights as I can before rehearsal starts.
Then I really need to show up at my full-time job before that description becomes a misnomer. So a full day Wednesday, then run out to pull another 4-6 hours of hanging lights and with luck will be ready to write cues...until 11 PM at night. And back up at 7 AM to once again follow the truck out to load in and perform that other show. (Then back to my day job for as many hours as I can cram into the afternoon before racing out to do the final tweaks before preview night).
Oh, and I'm still trying to shake a cold. I'm spending eight hours in bed, but I'm sleeping as little as four. Things are getting better -- at least I got the car fixed, and the fire alarm hasn't gone off again, but....!
(I type this as I'm trying to wolf down some basmati and something out of a packet before I fall into bed).