Hey!
From what I've seen in previous cycles, most winter vacation scheme and open day applications tend to open around August to September (based on the deadlines thread from this cycle, which you can find
here).
However, firms do seem to be opening applications earlier and earlier each year, so there may well be some opportunities that open before then. Additionally, firms tend to stagger their application windows throughout the start of the recruitment cycle, rather than all opening on the same day, so I wouldn't be too concerned about being away for a short period as I'm sure some applications will open later in September/October as well. In my experience, application deadlines are also usually a month of two after applications open, so you'll generally have a reasonable amount of time to get your applications submitted