Postgraduate student - when to apply for visa?

Hello all!  I will be starting a postgraduate program in the fall in the uk (still deciding on which school) and was wondering when I should begin applying for my visa.  What's the earliest I can apply?  What's the 4 monteh entry clearance window all about?

Are your referring to Tier 1 - Post Study Work or planning on studying in the UK?

I will be studying in the UK. Thanks for the links!  It looks like I can't have entry clearance post-dated for more than 3 months prior to entry.  Does that mean I can apply anytime, but I just can't get clearence for entry for up to 3 months before?  I'm a bit confused.

The international departments at the uni can help with these questions, as they deal with these visas daily. 

If you'll be starting in September/October, the best time to apply is the early summer- June or July. Getting into August they start getting quite busy, and because most UK courses start in early October, they are completely swamped in September. Don't buy a ticket until you have the visa, but you can ask for your entry clearance to be dated for around the time you want to go, e.g. entry clearance starts from 1 September. You have to enter within a defined period, so the post-dating is for people who want to apply in advance but don't want the visa to start too early so that they miss the window for entering.

and if you're applying for a student loan to help fund your studies, do so as soon as possible. processing a federal loan for an international school can easily take 2-3 months, or longer if you don't keep on top of things.