While firms who recruit on a rolling basis place different levels on emphasis on the time at which you submit your applications, I know many candidates (including myself) who progressed despite submitting very close to the deadline. As such, I think it makes sense to prioritize the quality of your application more than the timing. Since for most firms attending an open day will significantly improve your chances of progressing past the application stage in a VS/TC application, I think it is worth waiting for.
However, there are some caveats to this. Firstly, I do not believe it is feasible to implement this approach with a high number of firms. The reason is that most firms with competitive open day application processes will host the event in late October/November/early December. Most firms also have VS application deadlines in December/early January. As such, if you try to attend open days at many firms before writing your applications, you might end up completely overloaded come December, with several deadlines every week. This will likely mean you will have to compromise on the time you spend on each application and therefore on the quality of your research and writing - and my view is that quality of writing and research is the one thing you should not compromise on if you want to maximize your success chances. To ensure you can submit a high number of high quality applications, I think it is better to try to space out your efforts throughout the next three months. Thus, it is only feasible to change your application writing schedule around maybe 4-5 open days. This is why I would only wait for the open day outcome if this is one of those 4-5 firms that you would like to prioritize.
Secondly, there are some firms that are outliers in terms of how seriously they take the timing at which you submit your application - anecdotally, firms like Jones Day and
Gibson Dunn would be among them. Therefore, I think before you decide on whether to wait for the open day application deadline or not, it is worth having a look around this forum to see if the firm you are interested in is in this category. You should simply type the name of the firm in the TCLA Forum search bar, alongside some key terms like 'offer', and see if last year there were any people being progressed despite submitting the application later in the cycle. Alternatively, if you are comfortable sharing the name of the firm, you can post this question about it here. Someone who has had experience with applying for that firm might know more about the issue and get back to you directly.