How do open days work?
They are different depending on each company, but they are normally an introduction to the company. The general idea of an open day is to introduce you to the company and the daily runnings. Normally you will have to sign up for an open day, and much like a university open day, you will be toured around and shown what things are and even sometimes introduced to some members of staff.
Here is more information about what an open day is and why you should think of going;