Hi MissWorld21,
I attended an open day in Ireland. And the whole thing was spread over 3 days. The first day was the open day which started at 9am. People who didn't make it through the group activities were dismissed at about 12.00.
The day went on till about 6pm. Then we could choose if we wanted our final interview the following day, or the day after that.
Your open day is on a saturday. I don't know if they'll do interviews on the sunday! maybe someone else can answer that.
To be on the safe side, I suppose book your flight back on the Monday.
Hope you do really really well. Best of Luck