Hi Munei,
Open Days are really good opportunities to find out as much as you possibly can about a University. You will have the chance to see the campus/city (maybe for the first time), meet tutors and current students who are studying at the uni. There will be much more happening throughout the day too - I'd always recommend meeting the Student Union and finding out about sports clubs and societies as well as attending sessions on student finance and student life!!
Wishing you all the best,
Kate