Could the loss of Labor Day make that big of a difference in green-car sales? That's what some folks will likely be asking now that we know that September sales of hybrids, plug-in hybrids, battery-electric vehicles and diesels came in below 2012 figures.

The drop off between September 2012 and 2013 was worst year-over-year sales performance of the year so far.

With Labor Day falling in September last year and in August this year, last month's green car sales fell 1.8 percent from September 2012 to 46,523 units, marking the worst year-over-year sales performance of the year so far. Through August, green car sales had been up 26 percent, while the March sales decline of 0.33 percent had been the previous low point of 2013. More jarring was the 36 percent sales drop off between August and September.

As usual, the bellwether was Toyota and, specifically, sales of its four Prius hybrid variants. Prius sales in September totaled 15,890 units, down 16 percent from a year earlier. All four variants had declining sales, with Prius Plug-in Hybrid and Prius V wagon sales dropping 30 percent and 22 percent, respectively. The Japanese automaker didn't get much help from its other models, either. Camry Hybrid and Highlander Hybrid sales fell 17 percent and 21 percent from a year earlier, while hybrid sales at Toyota's Lexus division dropped 21 percent. Overall, Toyota's green car sales declined 12 percent from a year earlier to 23,425 vehicles.

General Motors' green car sales fared even worse, plunging 35%.

General Motors' green car sales fared even worse, plunging 35 percent compared to 2012 to 3,840 vehicles. Sales of the Chevrolet Volt extended-range plug-in fell 38 percent to 1,766 units, while sales of GM's mild-hybrid models such as Buick's LaCrosse and Regal and the Chevrolet Malibu plunged 50 percent. That more than offset the effect of the Chevrolet Cruze Diesel's 479 vehicles sold.

Offsetting some of these declines were strong performances at Nissan and Ford.

Honda green car sales also lagged, falling 5.8 percent from a year earlier to 1,159 units. While Honda Insight hybrid sales rose 22 percent from 2012, Civic Hybrid and CR-Z sales fell 18 percent and 21 percent from a year earlier. Additionally, low-volume models such as Porsche's two hybrid models and the Mitsubishi i electric vehicle were all down from September 2012. Volkswagen diesel sales were just about even with last year, at 8,675 units. Offsetting some of these declines was the strong performance of both Nissan and Ford. The Nissan Leaf EV just about doubled year-earlier sales to 1,953 units, while Ford's sales of its hybrids, plug-in hybrids and electric vehicles more than doubled to 5,926. Still, even that number was relatively disappointing, as Ford's year-to-date green car sales through August had been up almost fivefold from a year earlier. Overall, though, September sales of plug-in vehicles rose 23 percent from a year earlier to 6,977 units. Year-to-date, plug-in vehicle sales are up 77 percent to 53,279 units.