I've heard the end of the week, and also the end of the month, and even the end of the day. (Most of this was from a very green salesperson, it was his 3rd day and he was telling me how much insane pressure there was on him to sell cars because it was evening, on a sunday and also the last day of the month).
This makes sense, because sales managers set short term quotas for their salespeople like "you have to sell x cars today" (or this week) and presumably the dealership managers do the same thing (on a slightly longer weekly/monthly basis) for their sales managers.
However, in addition to that - yes, we are quickly approaching the 2004 model season. Most 2004s show up in september, I believe. So you'll see dealers starting to unload 2003 models very soon. There is a catch to this though - remember that 3-5 years from now, when you go to SELL your car, the model year will be very important. So in september, you might be able to buy an 03 for $20,000 or the 04 for $21,000 ... but remember that in 2007 when you are selling, the 2004 car will essentially be "a year newer" even though you could have bought it while 2003 cars were still on the lot!