Hi there,
In my experience the dates will alaways vary regarding opening dates.
There is however, one reoccurence that always seems to apply; most firms whether magic circle or regional firms tend to close their application process at the very end of July at midnight, that said, you should never wait until the last day to send your application, my tutors say that this shows a complete lack of organisation ad a lack of commitment to that said firm.
The opening dates vary - some firms open their process in december; this is to allow people to apply for both vacation placements and TC at the same time so you can be assessed for both at the same tie and be shortlisted for a TC if successful during the vacation scheme.
Others, more smaller firms open them in february or march.
In some cases firms take applications on a rolling basis by CV and covering letter or on the complete opposite have a very short window - one firm I looked at only had a one month application time; so as to only attract those who had their “hearts set” on the firm.
My advice wouldbe to research the sort of firms you are really interested in - either on the internet or by ringing the HR Department and asking what the situation is regarding their appication process.
Hope this helps.
Good luck with your studies. 
