Salzburg in Dec, I have questions!

Hi. I do not know about storing bags, so no help there. I also am not a Mozart fan and have never done a Sound of Music tour (never say never, though).

Your location looks fine and walkable to me for the main Markt and the fortress and the Altstadt shopping district. When I was last there in Dec 2019, there was a smallish Advent market near your hotel. This may be of interest when you are wending your way homeward in the evening - but is nothing to compare to the Cathedral district.

I enjoyed the Panorama Museum 1 minute away from the Cathedral, and even if you choose not to go into the museum, I believe there is store selling Salzburger Heimatswerk in the same building - for small gift items. Also, St. Peters Cemetery was a nice evening destination as I recall - it may close early-ish, say 6PM. The Getreidegasse shopping areas with its little passageways leading between streets is lots of fun. Beware, goods are fairly pricey in Salzburg, which never stops me from at least looking!

I did a simple google search: dec 3 salzburg and found a few events already on the calendar. More and more will pop up between now and then. When I was last there, I lucked into a free concert outside the cathedral in the early evening - nothing formal. I stood on the church steps. Just keep your eyes open for signage and you may encounter similar fun free things to do. You may benefit from getting a copy on this website or others of the Rick Steves Pocket Munich & Salzburg book. Even though I have been to Salzburg several times in December, I always refresh with a new look at RS books and then updated internet searches.

Enjoy.

https://www.salzburg.info/en/travel-info/infos/luggage-lockers_az_6689

We have reserved a night at Gästehaus im Priesterseminar,
Dreifaltigkeitsgasse 14, Altstadt for Sat Dec 3. The unusual location
looks really interesting to us, and the reviews are good. Has anyone
stayed there, by any chance?

I've stayed there three times, including once for 3-4 nights for Christmas markets. My last stay was about 6 weeks ago. Lots of atmosphere. Great courtyard. A traditional German breakfast that I rather enjoy.

Google tells me it's not far to the seminary, probably simplest to get
a cab to check in, with luggage.

I usually walk from the train station to the priest seminary, but I'm in my 40's and travel only with a carry on. It's a pretty good walk. My last travel companion (a few years older than me) had brought a fairly large roller bag on our trip. He was bitter by the end of the walk to the seminary and resolved to take a bus back to the train station when time to leave. If "slow of gait," I think a cab is probably a good plan.

Is there a good place to store bags at the train station? Either that
or we could leave them at the gasthaus, if it doesn't mean a lot of
backtracking.

You should be fine storing your bag at the Gästehaus. They have someone at the front desk from 8 am until around 5 or 6 pm, as I recall. Since the Gästehaus lies between the train station and the Old Town, you would actually being going out of your way to drop bags at the train station and then come back to the Old Town.

Recommendations for a good market in walking distance?

The main market in the vicinity of the cathedral is the star of the markets in Salzburg. Here is the non-updated English site.

There is a small market around Mirabelle Gardens near the Gästehaus. I was totally unimpressed. There is also a small market at the fortress. Again, not very impressive.

You will be there on a Saturday. The main market will be pretty packed that night. Try to do some wandering at the market during the day if possible. It is still worth walking around a bit at night at the market. The old town in general is decked out with lights and is very atmospheric at night, too, so strolling outside the market is rewarding, too.

The Afro Cafe at one end of the Old Town has great coffee and tea. It also has my favorite falafel in the world -- wonderfully spiced, atop a fresh and delicious salad.

Thank you Debbie--honestly a small market sounds great, especially at the beginning or end of the day. Being close makes it feel more festive! Would not have thought to check events, great idea. And probably time for some new guidebooks, as we "consumed" our Germany book this spring. (And didn't bring home the pieces!) Hmm, I wonder when the updates will be out...

@Sam, perfect link, thank you so much!

@Dave, delighted to hear of your experience! Repeat visits say a lot. By the time we get to Salzburg Hbf, we will have been up for...a long time. If we feel inspired we might walk, but a cab will work! Great orientation for us, too. Sounds like we made a lucky pick on location. I expected that Saturday would be busy, so we may wander around the edges if it's too much (or go to one of the smaller ones--first visit is going to be magical no matter what), but definitely looking forward to the decorated/lit streets. :)

Thank you again!