I did arrive on a Sunday evening on one of my visits to Ljubljana. There were restaurants open, which is more than I can say about Celje, a city I'd stopped at off the train on my way from Maribor. Celje was totally empty, like a ghost town, on a Sunday. Slovenia is a largely Catholic country - I assume that has a lot to do with it. A big attraction of Ljubljana

But as kja says, I'd base my decision on exactly what you want to see/do in Ljubljana and if things you want to do aren't available on Sunday, consider changing your plans. In many countries, Monday or Tuesday is the day museums are closed (if that's a factor). In fact, my "whole day" in Ljubljana on that same visit was a Monday and most of the museums were closed on Monday - plus it rained! I'm lucky I'd already been to Ljubljana before, though. I did visit a museum in Celje on that Sunday, so I assume museums are open on Sunday in Ljubljana too, but check the schedules of course.

(I was going to finish the first paragraph with "A big attraction of Ljubljana is the bustling cafe scene along the river and that may be muted on a Sunday.")

Thanks to both of you. Museums appear to be open, but I was worried things might be closed up. We were in southern France a year ago, and on Sundays ugly metal shutters were rolled down over windows. It totally changed the look of the towns.