Generally open days are best in year 12. Different places have them different times of year, but you generally want to go to them all pre applying (so if you will be starting vet school in sept 2015, that means applying by oct 2014), so open days autumn 2013/springsummer 2014.
eg: RVC has their open day may, so you would be ok to wait for 2014 for that - and will probably know a bit mroe about what q's you want to ask etc then, rather than going to far in advance.