I'm a current 3rd year astrophysics and cosmology student at Lancaster Uni and as I was applying to uni during covid, I went to 0 open days! It's really not necessary if you didn't get the chance to go, so don't worry about it at all, it's more for prospective student to get a feel of campus / uni facilities / see accom / features of the uni itself to see if it's something that would appeal to you and if it's somewhere you picture yourself spending the next few years in.
If you're wanting to look at unis and have tours, a lot of them have vitrual tours either on their websites or on youtube. Something else I did was see if there were students that made videos (tiktoks or youtube videos) that were "day in the life's" at unis I wanted to go to, doing courses I wanted to apply to. You get a really good feel of places through a students perspective rather than the uni, so I'd highly recommend doing that if you're wanting to explore that.
I hope this all helps and wishing you all the best for your future! If you have any other questions, feel free to ask.
-- Arya (Lancaster University Student Ambassador)