If you're thinking of shopping for a new car this weekend, here's some advice: Call first.

Because if you are looking for a car in the Washington area on a weekend, you're going to have to steer clear of Virginia's "day of rest" requirements, which allow car salesmen to declare "Never on Sunday" if they so choose.

And you'll also have to map out Maryland's "blue laws," which deem it morally permissible to sell cars in Howard, Montgomery and Prince George's counties on Sundays but find it a violation of the Sabbath to sell them in the other 21 jurisdictions.

It took me some time to figure this out after a reader asked: Why can't you buy a car on Sunday?

I put the question to one representative of an automobile dealers association who came close to telling me I didn't know what I was talking about. Drive down Rockville Pike on a Sunday, he insisted, and you'll find it's a very busy day for car sales.

So I started calling auto dealers in Montgomery County, who assured me that of course they were open on Sunday.

Then I called Brown's Honda City in Glen Burnie in Anne Arundel County, which assured me that of course it was not open.

"We're closed," salesman Tim Eubank told me. "Everyone knows we're closed. It's the blue law. We don't have a choice."

Come to Honda City on Saturday or Monday, Eubank said, but not on Sunday. Shop online 24 hours a day, he said. All you have to do is visit the Web site to see what's in stock. You can even figure out the value of your trade-in with the Kelley Blue Book on the Web, on a Sunday. But whatever you do, don't expect to find a salesman on the lot on that day.

When I called David Reynolds at Alexandria Toyota, he assured me that you can bet on the dealership being open every Sunday from noon to 5 p.m. "It's customer service, 100 percent," he said. "They expect to be able to shop on Sunday. It can be a busy day for us."

Only a few minutes later, Pohanka Chevrolet in Chantilly was telling me that they are open only two Sundays a month -- the last two -- in an effort to satisfy both customers and employees who need a day off.

On it went: Fairfax Jeep, closed. Fairfax Honda, open.

At this point, I started leaving messages with county attorneys. Richard H. Melnick, associate county attorney in Montgomery, called me back with his legal books open to an intriguing passage.

Section 18-101 of the Maryland Business Regulation Article of the Maryland Annotated Code says that in general, retailers may do business on Sundays. Subsection d lays down the law for all Maryland car dealers EXCEPT those in Howard, Montgomery and Prince George's.

That section says that a new- or used-car dealer may not sell, barter, deliver, give away or offer for sale any car on a Sunday. The next section stipulates the time professional sports teams may play on Sundays (after 1 p.m.) and gives no indication whatsoever why only the chosen three counties can sell, barter, deliver or give away cars on Sunday.

Why should I have been surprised when I discovered the reason was political?

It turns out that when Maryland dropped most of its blue laws in the 1980s, car dealers in most of the state were not interested in joining the movement toward Sunday sales. Car dealers in Howard, Montgomery and Prince George's felt otherwise. Arguing that they had to compete with dealers in Northern Virginia, who were open, they managed to get an exemption from the old ban. In 1998 and 2000, some car dealers eager to open on Sundays lobbied the Maryland legislature in favor of a bill that would have lifted the ban entirely. They had no luck, with most of the state's auto dealers backing the status quo.

The situation in Virginia is just as unusual, as Keith Cheatham, director of government affairs for the Virginia Chamber of Commerce in Richmond, explained it.

"Our blue laws were repealed in the 2004 General Assembly," Cheatham said. "Unfortunately, they also repealed a number of exemptions that had to do with the day of rest."

Though the blue laws and the day-of-rest laws were addressed at the same time, they are two different laws.

"The day-of-rest law applies to a number of folks," Cheatham said. "It says that everybody in Virginia who requests it may have a day off during the work week. Then it goes on to say that if you are not in management, you can request Saturday or Sunday."

The day-of-rest law has been in effect since the 1600s, but in recent centuries, as the economy has changed, exemptions have been added. There are now 18 exemptions, Cheatham said, including hospitals and entertainment businesses. Most retail stores are not exempt, he said, and neither are car dealerships. So those car dealerships that are open may be staffed by management or they may be staffed with employees who haven't insisted on their Sunday off.

Cheatham said as far as he knows, the Virginia dealerships that are closed have not closed in response to the day-of-rest law, which has not been widely observed. Instead, he said, they probably have made a business decision that six days a week is enough.

"People do need a day off," he said. "People spend time in church with their families, and maybe they have no interest in going to buy a car on Sunday. Some auto dealers may have looked at the numbers and decided it's not economically effective to work on Sunday."

So if you're dedicated to Sunday shopping, better make the Yellow Pages your first stop. I called a couple of dozen dealers without discerning any pattern before I hit the law books. In the District, I discovered mostly small dealers who chose not to open, but that's not saying there aren't dealers who have the lights on. My fingers may not have walked far enough to find them.

If you're a shopper like Cheatham, maybe the Sunday closings are a good deal. That's his favorite car-shopping day.

"When I'm looking to buy a new car, I enjoy going to look at lots on Sunday when no one is there," he said. "I just want to look. I don't want to be bothered by the hustle and bustle. Then I'll go back when the salesmen are there."