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You've heard all the advice about the best time to buy a car, and now you're eager to bounce into a showroom and snag a killer deal. But it's not that easy:

There's no particular part of any month when you're guaranteed to get the best bargain.

There's no one day of the week when dealers will be most open to lowball offers.

There are just as many people who think you'll get a good deal in the early morning as those who think the day's end is when the salespeople get sleepy, sloppy or desperate to make great deals.

All that, however, doesn't mean you can't save by gauging the timing of your transactions and being aware of market forces. And keep in mind that the rules are different for new cars versus used cars.

The tax-time used-car bubble

Steven Lang, who runs a dealership and an auto auction in the Atlanta area and writes for TheTruthAboutCars.com, says the used-car market is particularly sensitive around the time Americans are getting their tax refunds.

"From mid-February through early June, the auto industry goes through an event called tax season," Lang says. "Folks receive their refund checks from the IRS, which in turn creates a nice little bubble of activity in the used-car industry."

As evidence of this bubble, Lang cites the prices paid for used cars at auction during March and April. "Compared to three months (earlier), I would estimate cars sold between $700 and $1,500 higher than what was then the market price," he says.

It makes sense that with tax refunds in hand, many people have the chance to put a down payment on a used car or buy a used car outright. So that's when a lot of buyers are out there. And when there are a lot of buyers and supply doesn't increase, prices go up.

"I'd say used-car dealers have done two-thirds of their business for the year by the Fourth of July weekend," Lang says. "The best time to buy is October or the beginning of November. There are no holidays then. No one is getting their bonuses. And the inventory is depleted. Wholesale prices are down then, too. It all reflects the retail climate."

The new-car market isn't as sensitive to the tax-refund bubble. In fact, according to data from Edmunds.com, during 2010, new-car transaction prices showed the greatest discounts in March and July. April and May were close behind.

The end of the month may not matter

"There's some truth to the idea that new-car dealers may make better deals at the end of the month," says Phil Reed of Edmunds.com. "But people shouldn't think that a little tip like that alone will get them a great deal."

New-car sales are greatly influenced by manufacturers -- both by what incentives and financing programs they have in place and by production rates and the resulting size of their inventories. Sometimes such incentives or other sales promotions run out at the end of a month, and it may be in a dealership's interest to make a better deal then.

But a lot of buyers believe they can get better deals at the end of the month. That means more people are chasing cars at the same time. And more buyers chasing the same number of cars means dealers have less incentive to offer bargains.

Beyond that, there's nothing that says manufacturers have to end their incentives at the end of the month; they can end them any time they please. The trick is to know what the incentives are and when they're in place.

Websites such as Kelley Blue Book and Edmunds.com track incentives and inventories and report them. Use those tools.

Rainy daze

"People who think they'll get a better deal on rainy days and such are just being silly," Reed says. Dealers don't throw their business sense overboard just because it's a little damp outside, and they can wait out a cloudburst.

"It's easier to shop when there's nobody else on the lot," Reed says. "So being the first customer of the day may not be a bad idea. But that's not going to necessarily save you a lot of money."