Actually there are two school's of thought about this.
1) Do it a few days (up to a week in advance) let it fully ferment and rebuild up it's glycol reserves. Refrigerate pour off the spent wort leaving only enough to make a slurry of the yeast and then pitch.
2) Do it 24 hours or even the morning of the brew and pitch it when its reached high krausen that day.
Nobody here seems to talk about #2 saying that you need at least a few days for a starter.
Im getting my info from the Pope himself Jamil Z. (Ive been listening to his show a lot of late.) He says he usually does #2. He does #1 with lagers because the starters are fermented warm that and the fact that theyre so large they would produce noticeable off flavours.
<<starters reach their maximum cell density within 12-18 hours, which sounds about right to me. Some sources suggest that maximum cell densities are not achieved until 24 hours and others as much as 36, but the return on waiting that long is minimal. Let's just say that the bulk of the yeast growth is done by 12-18 hours. I like to pitch starters while they're still very active and as soon as the bulk of reproduction is finished, usually within 8 to 18 hours. This is really convenient, because I can make a starter the morning of the brew day or the night before the brew day and it is ready to go by the time the batch of wort is ready. There is no need to make a starter a week in advance, because I pitch the whole starter, liquid and all (up to a certain size of starter). Yes, you can wait longer and completely ferment it out so you don't have to pitch the liquid, but if you're going to do that, you should use a larger starter and allow the fermentation to go complete cycle over several days, chill, decant the beer and pitch just the yeast. If you're making a smaller starter, it is better to just pitch the entire active starter within about 6 to 12 hours of pitching the yeast into the starter.>>
http://www.mrmalty.com/pitching.php
Rudeboy