Most have them in June/early July and then September/October.
Some get booked up quickly, but it's also worth going to "local" ones even if your child isn't interested in going to it, as a lot of the information and "talks" are generic, i.e. student loan finance and you can also gain "general" knowledge about the types of courses, structures, accommodation, etc.
Our first was a year "early" (i.e. a year ahead of the year before starting), at our nearest Uni, and DS found it enormously helpful in a kind of "toe in the water" way to get a feel of Unis, the open day structures, accommodation/lab tours, etc., and it actually ignited his interest in the whole Uni/Application process.
Then when you come to the "booked up early" Unis you can concentrate on what really matters and what is specific to that university and not waste part of your day in generic talks, so spend your time in the department, doing dept tours, doing specific subject talks, talking to existing subject teaching staff and students, etc.
It was a completely different Uni where my DS learned most, as one of his "third/fourth" choice uni dept "dept talks" did a really good overview of different "options" within his chosen subject that he'd not considered and didn't know even existed. He'd been blinkered into sole Maths degrees or "Maths with Physics" or "Maths with computing" etc., but this particular Uni's Math's lecturer also started talking about other "Maths" related options such as Natural Sciences and Financial Maths, which both were of far more interest to DS than either a pure Maths degree or a specific "Maths with x" degree. He was excited after that talk and did loads of research to find which Unis did Natural Sciences and which did Financial Maths, and then narrowed his search down to those which completely changed his Uni preferences so we had to cancel some open day bookings we had made and change our plans!
The whole process evolves and I think most students end up with completely different preferences as to subjects/Uni once they start doing open days when they finally get their heads out of Youtube and online gaming and actually start doing their own research, which is often ignited by that first Open Day.