start of the month... they have targets to meet so if they have hit them in first week or so then after that the dont need to try.. the run up to summer and xmas as well.. low sales due to holidays and xmas pressies
The end of the month as salesmen are trying to make numbers and will be more willing to ensure they get the sale...this is what I have done for all my cars and it has worked in my favor for both used and new that I have bought!
All of this is good advice, but to be honest unless you haggle properly and do your research before you go anywhere near a showroom, it doesn't matter what time of the month/year you're buying.
Go through autotrader and get some average prices for the kind of car you're looking for. Even go so far as to take printouts of some of the bargains along for more haggling power. Have a set price in mind before you go to the showroom, and don't go above it. Remember, the salesman wants to get as much out of you as he can, and give you as little as possible. Always be prepared to walk away, there's no such thing as the perfect car for you, you'll get the same elsewhere in a weeks time, trust me.
Showrooms generally build in 10-15% mark-up in price minimum when they buy in stock, so offer them 10% less than the sticker price as a starting point in the haggling, but don't ever pay any more than 95% of sticker price. Likewise if you have a trade-in, try and haggle at least 5% more out of them for it, they will try and get it for rock-bottom to start with.
A salesman will never give you "best" price unless you do a lot of haggling. Remember, they are not your friend, and they do not have your best interests at heart. They want to make a profit and sell you a car.
Also, try and haggle for little stuff to be thrown in gratis. Never buy an untaxed car, I expect that as standard if I'm buying a car from a trader. Try and get them to throw in stuff like warranty, maybe a few services, a tank of fuel etc. you get the picture. If they won't, then haggle them down even further on the price
Bottom line, be ruthless, and walk away if you don't get what you want. Don't settle.