Before you book a university open day make sure that you have checked the entrance requirements for the course that you are interested in and that they match your predicted grades.
Most universities have open days on Saturdays. You should book these before you take time out of school to attend. There's no point knowing all about your prospective university if you miss out on the learning that will get you there. You are allowed up to three days absence from school in order to attend open days. These should be taken in the last two weeks of the summer term which is when most of the midweek open days are scheduled. Your attendance at these must be approved by your Director of Studies before you go.
Opendays.com has a complete timetable and booking system.