Four adults will be traveling from Munich to Regensburg for a day trip. Due to the days we will be in Munich, we will only be able to go to Regensburg on a Saturday or a Sunday. We were leaning toward going on a Sunday. as we arrive on Friday, but understand the shops will be closed. We are planning on seeing the sites and Christmas Mkts so this may not be an issue. Will we miss much if we go on a Sunday?
There's a lot more to do in Munich on Sundays (after 1300) than in Regensburg.
I am not directly answering your question, but can say that I have been to Regensburg twice, once in the 80s when I lived in Germany and the second time a few months ago when we took a Danube cruise from that city.
The city is rather small and can be done on a day trip from Munich. You can take the trains, which will be regional. Regional trains stop frequently and are a bit slow, but suggest leaving early.
Be sure to have lunch at the old bridge over the river where there is a bratwurst restaurant (fantastic) that is several hundreds of years old. Some say that the restaurant is in the building that provided food for the workers that built the ancient bridge.
I don't think so unless you really wanted to shop. Souvenir shops will be open along with Restaurants and Cafe. Don't see your dates, but you'll probably spend your time at the Regensburger Christkindlmarkt, Churches, small lanes, Steinerne Brücke, Wurstkuchl etc.
I would suggest you contact the tourist bureau. They replied promptly to a question I had. We are taking a day trip from Munich in December also.
I‘m a huge fan from Schloß Emmeran. It‘s so big and nice. Schloß Emmeran offers a Carriage Museum. Worth a visit, too.
https://www.thurnundtaxis.com/discover/visit-st-emmeram-palace/book-a-tour
The Christmas Market has a special atmosphere.
https://wm-tut.de/start-page
And a very nice Basilika is beside, there.
All possible to visit/book on a sunday.
A nice place for a break, especially if the weather is not good, here is a tip. The restaurant is next to the castle
https://www.brauhaus-am-schloss.com/
Or near the Dome https://www.hotel-bischofshof.de/3182-revision-v1/?lang=en
Be sure to have lunch at the old bridge over the river where there is a bratwurst restaurant
Wurstkuchl (Sausage Kitchen) https://www.atlasobscura.com/places/wurstkuchl-sausage-kitchen
Last month I did a day trip from Munich to Regensburg. One day was plenty to walk around and see the streets and bridge. I went into the main church in the center, a small museum by the bridge, had lunch and supper, went to a coffee shop, walked around the interesting medieval type streets (never bombed so original, unlike many German cities), and walked across the bridge to the opposite bank which had more cafes and coffee shops. I’m sure all these things would be open on a Sunday. I don’t know what shopping a tourist would do there. Shops that sell everyday things like clothing, sporting goods etc might be closed but who cares, you can buy that stuff anywhere.
The only problem was the train going back to munich was an hour late, but not a big deal in the grand scheme of things.
Here are the opening times for christmas markets on sunday in Regensburg. All in a good walkingdistance. I think, you will not miss the other closed shops. Enough to see and shop :-) in a day.
https://tourismus.regensburg.de/en/events-and-tickets/christmas-in-regensburg