It's a pretty good idea!
I'm heading to one or two open days while I still have my summer left, and probably a few more in September/October to start off with. I think its important to have a good idea of which universities you'd like to apply for as early as possible, so that you can use it as a motivation for studying, and also so that you can help tailor the appropriate activities and enrichment you ideally need to be doing, so that your personal statement can be filled with lots of wonderful things!
The specific course content of universities is also hugely important when making decisions, as after all, you'll be doing it for maybe 3/4 years. I guess another major reason is that we're probably going to be a lot less busier and under stress towards the start of the year, and you may be waiting a while for the next university open day if you miss the earlier ones.