Not going to open days won't be 'bad', as such. A large number of people get tricked into think it's representative of what uni life is like, but in actual fact, open days are days when there's virtually no actual students at uni, and everything is targeting high schoolers.
It's a good idea to go to a uni you're interested in, and see what the vibe is like, but you can do this anytime during semester and get a more realistic view of what uni life is really like. There are also opportunities to book appointments with professors of the subject area you're interested in.