If you are down for an elcs and really want it (i.e don't want to go anywhere near a 'trial of labour' scenario, or have been advised against it etc) then you go in AS SOON as you think ANYTHING may be kicking off.
The advice is the opposite for labour aiming for a vb, which is to stay at home as long as possible, and only come in when you have to.
if you do go into labour before your elcs date, they will aim to do the cs earlier, is all - but to give them best opportunity to fit you in (and that means getting you into theatre as a 'non-emergency' or 'semi-emergency', which is a good way of putting it!) then they want to get you on the list asap. having to fit you in at short notice is much more troublesome for them. It's just because as a non emergency, you can be bumped down the list if a life or death emergency comes in, which of course makes total sense.
A very small number of women labour so fast that they are too far progressed by the time they get to hospital for a cs. Don't be alarmed, it's something that hardly ever happens, according to my consultant mw - but it's one reason not to hang around waiting to see what happens at home.
Some advice from my mw - make sure that it is in your notes that you are to go to cs if you do start labour before your date, and make sure any mws you talk to if you ring the labour ward don't persuade you to 'have a warm bath/take paracetamol/see how it goes'. It's just advice that doesn't apply to you.
My waters went nearly 2 weeks before my cs date - I went into hospital, had the cs next day. All was fine! (i was over 37 weeks)