The New York City area carries the highest overall U.S. per diem for 2011: $340, split between $269 for lodging and $71 for meals and incidentals. (Jennifer S. Altman / For The Times)

The next time you wonder where to take your next vacation, don't forget to ask when. The right answer could save you hundreds of dollars.

Depending on the season, the elite retreat of Martha's Vineyard, Mass., can be cheaper to visit than Chicago. An overnight trip to California's Yosemite National Park area may cost more than a night in San Francisco. And the Vail, Colo., area can be pricier than just about anywhere in the country.

I learned this from an unlikely source: per diem rates set by the U.S. General Services Administration for fiscal year 2011, which starts Oct. 1. With a few exceptions, these rates cap the total that a federal employee traveling on official business can spend each day for lodging, meals and incidentals such as tips and local transportation. The caps vary by destination and often by date too.

The per diem is not a bad guide to what you might spend on vacation, even though federal officials don't endorse using the figure that way.

"GSA can't speculate on what leisure travel rates are," said Jill Denning, per diem program manager for the GSA's Office of Governmentwide Policy.

But like many vacationers, the per diems shoot for the middle range. (You can see the rates at http://www.gsa.gov/portal/category/21287.)

Each year, the GSA contracts with a vendor to determine the average daily room rate for mostly midpriced chain hotels, such as Hyatts, Marriotts and Sheratons, in each of the nearly 400 U.S. destinations federal employees visit most frequently. (Outside these destinations, the U.S. lodging per diem, $77 for 2011, is based on the nationwide average.)

"We strip out the economy and the luxury properties," Denning said. So no Ritz-Carltons, at one extreme, or Motel 6s, on the other.

The meal allowance, also based on a vendor's survey, is moderate too.

"We call restaurants in a local area, and we simply ask them the price for breakfast, lunch and dinner," Denning said. "We don't do fast-food restaurants. We're not looking at the top tier."

Meals-and-incidentals per diems are the same year-round ($46 to $71 for 2011, depending on the destination), but lodging per diems are all over the calendar — and the map.

The New York City area carries the highest overall U.S. per diem for 2011: $340, split between $269 for lodging and $71 for meals and incidentals. But it's only on top Sept. 1 to Dec. 31. For stays during most of the summer, Nantucket, Mass., commands a higher per diem ($304) than the Big Apple ($262). During much of the winter, so does Eagle County, Colo., home of Vail ski resort: $332 versus $263 for New York.

The gyrating rates reflect, in part, visitor numbers. In fall and winter, New York is besieged by business travelers, theatergoers and holiday shoppers. Nantucket, like many beach resorts, peaks in summer. And Vail packs in powder hounds in winter.

Instead of fighting these ebbs and flows, make them work for you. Some off-season opportunities:

Ski resorts in autumn: Fall colors can be spectacular. But the season's other appeal is deals, which recently included a third night free in Steamboat Springs, Colo., and Park City, Utah. In autumn, GSA's per diem for Vail plunges to $176, about half the winter rate.

Santa Barbara in spring, winter or fall: This self-described American Riviera is far more serene outside summer, with mostly mild weather. Even at a per diem of $209, as opposed to $238 in July and August, it doesn't come cheap. But its great cultural venues and frequent festivals are priceless.

New York City and Washington, D.C., in summer: You may swelter, but you'll also find swell things to do. In New York, the parks are abuzz with festivals, free concerts and Shakespeare in the Park. In D.C., you can wander Smithsonian Institution museums for free. Per diems in both cities reach their nadir in July and August.

Of course, sometimes you can't escape hefty prices by switching dates. At $251, seaside Santa Monica commands the richest per diem of any place in California — year-round.

jane.engle@latimes.com