With dc1, we did all our open days in the Autumn Term of Yr13.
I say 'all' - we did 4. I took him to 2 and dh took him to 2.
It's worth seeing if they can jump in with a friend going to the same one - I took a couple of dc's mates on our visits - no skin off my nose, I was going anyway, but their parents were really appreciative (one was a Friday which I can do but can see that not all working parents would be able to do).
If you can, I think it makes sense to go and visit. They are going to live there for 3 or 4 yrs after all. Out of the 2 I took ds to, you could just see the difference in how much he liked the 2nd one (where he ended up) rather than the first, although that was not what he thought from looking on-line/ at the prospectus. I guess it's like looking round a house, you just get a "feel" for the right one for you.