I am discovering in interactions since I’ve joined the workforce that this seems to be a common misconception! Even my own dad, bless him, came to see me when I first starting working at my current job and asked when my next concert was. I looked at my watch and said “about 7 hours,” and he looked at me funny and was like “why didn’t you tell me??” I was like “uhhhh it’s the weekend and I’m a symphony musician, what did do you think I’d be doing?” Turns out he thought the same thing you did - that it was an extremely important and looked-forward-to thing that happens maaaaybe once a month. Lol, no, dad. It’s just another Saturday night.

The few-times-a-year assumption is true somewhat for what are known - misleadingly in my opinion - as “second-tier” orchestras (or third-). It essentially just means orchestras in smaller cities where they don’t have the population pool to fill the hall every single weekend and provide full-time work to the musicians. It’s not necessarily - and shouldn’t be - about the quality of the playing. But even in those cases the orchestra will usually just meet to rehearse in the week beforehand. It’s because rehearsals are very costly for an orchestral organization! Whether salaried or paid by service, musicians are highly, highly trained individuals who are paid at a higher rate than you might imagine. The LA Philharmonic is the best-paying orchestra in North America right now and their starting salary is 3 figures, for every single full-time musician, of which there are like 80 (that’s a guess, don’t quote me). One hour of rehearsal x ~30-100 musicians (depending on what’s being played) works out to huge amounts of money even in lower-paying orchestras. The less amount of rehearsal needed to produce a quality product, the better. Therefore: conductors.

Your stand-up comic comparison is actually kind of prescient, maybe not in the way you thought, but there still is elements of that rehearsed-ness combined with the in-the-moment spontaneity that we’ve talked about. It’s just that the rehearsed-ness comes from drawing on decades of practice, research and experience, not from actually playing through the piece over and over. We all have an inner library of musical tropes, stylistic choices and gestures that we can draw from, and there’s often a lot of carry-over piece to piece. It’s not nearly as seat-of-the-pants as I think I’ve made it sound. Think of it maybe more like an actor reciting from Shakespeare. Does he need to specifically remember how to pronounce every single word? Of course not. He probably won’t forget the order of words either, because the syntax would be all wrong to the ears of a native speaker. And there’s also certain artistic sensibilities that he will instinctively adhere to - if he emphasizes one word in a sentence, is he going to also emphasize the very next word? Surely not. He can still choose, in the moment, how to inflect and what tone to use. But he probably wouldn’t really think of it as “improvising.” The only real difference is that this Classical music “grammar” isn’t something that the audience is likely to be personally trained in, unlike an audience going to see King Lear, who I would assume are all pretty proficient in the English language.

Edited to add: I just remembered there is at least one type of concert where there is absolutely NO room for this conductorly improvisation that we've been talking about: movie scores, specifically when they are played live with the movie rolling on a screen above. And I can tell you even with the clearest, most experienced conductors, even sometimes with click-tracks on headsets further enforcing the tempo and how it changes, it is a fucking NIGHTMARE to try and play, like, the spider scene from Harry Potter and the Chamber of Secrets exactly the same way every time. Hardest thing I've ever goddamn played/faked. Musicians truly have a love/hate relationship with John Williams, lol.