Based on non-Covid experience:
If I recall correctly, there were only 2 or 3 a year. They were... formal-ish? Most people wore pretty smart clothing, suits or nice dresses, few of us even went full black-tie. At Sherwood, the atmosphere wasn't a whole lot different. Still the dining hall, and while we did have decorations, live music, and three courses, it was still very much a catered accommodation dinner. Well, except for the drinking; they opened up the bar (which in Sherwood is never open for anything else), and most people had whatever they considered a reasonable amount of wine. People went out afterwards, as is the way. I think there may have been an official after-party, but I'm not 100% on that. One time we had a cotton candy machine too; I ate a damn lot of cotton candy.
I also went to one Broadgate Park one. I was living in Cloister, and since Broadgate is huge while Cloister and Albion are tiny (and all self-catered), they share one. That one felt pretty different; since it's self-catered accommodation with a lot of people, we actually had a different venue. I would say the food and entertainment was a bit better. Also, for some reason there was a cheese and biscuits station? I think I was the only person there who was actually excited by that.