I completely agree with
@Jessica Booker's advice - waiting to attend an open day and then mentioning that in the application is very unlikely to hurt your chances. Firms generally prefer applicants who attended open days and as such, although the application process might be rolling, they will not hold the later submission date against you. That would undermine the purpose of the open day, since an open day at this time of the year is organised to provide applicants with insight for this application cycle, not the next.
The only other thing I'll add is that while I think waiting to attend open days for each firm does generally lead to better odds for the success of each individual application, it does not necessarily translate into better odds at overall success in securing a TC. The reason for this is that employing this strategy for a high number of firms can make your application schedule administratively difficult.
Let's say you aim to apply to 20 firms this cycle, and let's say you would want to attend an open day at each before submitting a VS application. Firstly, this means that besides 20 VS application, you will need to take the time to write 20 open day applications, which is a significant increase in workload. Secondly, and more importantly, most open days you can apply to now will only be held between November-December. That is the exact period for most VS application deadlines. While there might be at least a two-week gap between the date of the open day and the deadline for VS applications at each individual firm, these periods are likely to overlap for different firms. Thus, if you do not submit some applications early, you could find yourself in a situation where you have 5 deadlines in a week. This can lead to a significant decrease in the amount of time you can invest in writing each application, which will in turn lead to a significant decrease of your chances of success. Alternatively, should you not want to compromise on quality, you may be forced to forego applying to some firms you wanted to apply to - which once again will reduce your overall chances of securing a TC. Hence, my advice would be to only wait to attend open days at a small number of firms you are particularly attracted to - around 4-5 is what worked for me.