I went to some October open days at the start of my AS course. I would recommend going as soon as possible, because if you get there and don't like some then you can go to some in and around April when the next lot are.
Go to as many open days as is humanly possible, even for Unis you have no intention of applying to. It gives you a good idea as to general standards and what type of uni you'd like to go to.
I did it from when I started college. Check the dates, quite a few had open days February and then April/May/June, so waiting for the summer holidays (end of june and august) may make you miss them.
Just pick uni's you want to look at, whenever the date is, go and look. Earlier is better, so you can go back if you want to double-check.
i'd say go to the open days as soon as you can, you may also like to vist again after you have submitted your application to ucas just to make sure nothing has changed.
I started going about spring/summer time of Year 12, and then am going to see one the first week of term during A2 level.