Why is Labor Day weekend considered a great time to buy a car?

Traditionally, automakers updated their cars almost every year, and the new models were launched in the fall. That made dealers extra-motivated to sell the last of the outgoing models before the new ones arrived. 

Automakers don't update their cars every year anymore. But the changes, when they do happen, are usually more extensive than a few styling tweaks. That can make today's dealers even more willing to deal when a revised model is incoming.

The trick is to know which models are being revised. Analysts at car-shopping site Edmunds say that at least 31 vehicles are scheduled for an overhaul for 2016. Deals on the 2015-year models that are still left on dealers' lots should be quite good. Here are six choices that look especially promising.

2015 BMW 3 Series
BMW's (NASDAQOTH:BAMXF) 3 Series has long been the gold standard among compact luxury sports sedans. In recent years, rivals have closed the gap. BMW is releasing a revised version for 2016 in a bid to stay at the top of the heap. But the 2015 model is still a very good car, and dealers are looking to clear out inventory. 

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2015 BMW 3-Series. Source: BMW

Edmunds says that actual selling prices on the 2015 3 Series have already dropped by $1,500 or more in some parts of the country. BMW is stepping up with special lease deals and "cash back" offers on some versions, and many dealers will have low-interest financing available if you qualify. 

2015 Chevrolet Volt
General Motors' (NYSE:GM) innovative plug-in hybrid sedan will be all-new for 2016. The new model, which will arrive at dealers soon, comes with real improvements -- but the outgoing model is still a very well-regarded car. Over the course of its life, it has had some of the highest customer-satisfaction ratings in the business. Owners say that it's quiet, comfortable, reliable -- and, of course, super-cheap to operate, especially if you remember to plug it in every night.

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2015 Chevrolet Volt. Source: General Motors 

Like most of its hybrid and electric rivals, sales of the Volt have fallen sharply as gas prices have dropped. Edmunds says that market prices have already dropped by almost $4,000. Dealers with 2015s on hand will be especially motivated to deal, and GM is making plenty of incentives available. 

2015 Toyota Prius
As with the Volt, sales of Toyota's (NYSE:TM) acclaimed hybrid have fallen as gas prices have risen. With an all-new 2016 Prius inbound, Toyota dealers are especially motivated to move the 2015s they have on hand, and Edmunds says transaction prices have already fallen by as much as $3,000. 

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2015 Toyota Prius. Source: Toyota

A bonus: Toyota is offering interest-free financing for up to 60 months to 2015 Prius buyers with good credit. 

2015 Chevrolet Equinox
GM's popular, well-built five-passenger crossover SUV is getting an update for 2016. But the 2015 makes a terrific family car: It has a smooth ride, lots of storage space, and a long list of useful features. 

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2015 Chevrolet Equinox. Source: General Motors 

The Equinox has been selling very well -- but with 2016s on the way, there will be good deals to be had this weekend. GM is stepping up with attractive lease deals and low-interest financing offers on some versions, too.

2015 Honda Civic

Honda's (NYSE:HMC) Civic has long been a popular car for folks on a budget. It's simple, reliable, roomier inside than you'd think, and likely to last a long time. The 2015 model has all of those traditional Honda virtues -- but it's been losing business to snazzier-looking rivals.

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2015 Honda Civic Sedan. Source: Honda

Honda is responding with an all-new Civic for 2016 that features more dramatic styling. But there's nothing wrong with the 2015s if you're looking for good, dependable transportation that will hold its value well over time -- and again, Edmunds sees some good deals happening.

2015 Honda Accord Sedan
Last but definitely not least, the Accord has long been Honda's flagship model in the U.S. The 2015 edition is a fine-handling sedan that's surprisingly big inside. That's especially true in the back seat, where adults -- or tall teenagers -- will find ample side-to-side space and very generous legroom.

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2015 Honda Accord Sedan. Source: Honda

Accord sales are down sharply this year, and Honda is giving the model an overhaul for 2016. Edmunds is seeing lots of deals below invoice, and Honda is offering some incentives. Take special note of this one: The 2015 Accord is a very nice, roomy car that is inexpensive to maintain and likely to have outstanding resale values.

John Rosevear owns shares of General Motors. The Motley Fool recommends General Motors. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.