In short, yes, you usually can visit the student halls at an open day. Universities have different ways of organising their open days but almost all of them will make some provision for their prospective students to see the student halls on their open day.
Most universities give a guided tour at allotted intervals or at a set time at some point during their open day.
Here’s more about university open days and student halls:
If it is first come, first serve then find out when you can apply for accommodation and make sure you get your application in on time. Smaller universities have hundreds of students competing for rooms so don’t leave it until the last minute to find somewhere to live.
You’ll also pay more for catered accommodation so if you’re capable of feeding yourself without causing a fire or giving yourself food poisoning then you’re probably better applying for self-catered halls.
Location is another important thing to consider and will affect the cost of your halls. Student halls on campus are convenient but you’ll have to pay for being so close to your classroom. Some universities are spread across a city and you could find yourself in halls on the other side of city to your lectures, but you will pay less money for your room.
Ask this question on the open day and see what the university has in place for students who don’t get into halls of residence. Chances are you’ll need to find private rented student accommodation.