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Airplane Tickets

When to buy: On a Wednesday, 21 days (or a couple of days earlier) before your flight.
Why: Airlines make major pricing changes (and run fare sales) every week, typically on Tuesday evenings and Wednesday mornings. About 21 days out from your flight, you'll see plenty of deals out there as airlines scramble to fill seats. Don't wait much longer, she cautions; prices jump significantly from 14 to seven days ahead of departure.

Broadway Tickets

When to buy: Hours before the curtain rises.
Why: How does a $25 front-row seat to the smash musical "Wicked" sound? Several musicals offer same-day ticket lotteries that offer up orchestra seats at inexpensive prices. If you'd rather not gamble on getting a seat, wait in line at the famous TKTS booth in Times Square. There, you can get tickets for hit musicals for up to 50% off. On a recent night, prime seats were available for "Hairspray," "Rent," "Sweeney Todd" and "Beauty & the Beast." (For the right times to drop by TKTS, and other ways to save, see our column A Midsummer Night's Dream.)

Cars

When to buy: Weekday mornings in September.
Why: By September, all the next year's models have arrived at the lot, and dealers are desperate to get rid of the current year's leftovers, says Phil Reed, consumer advice editor for Edmunds.com. It's the prime time of year for incentives and sales, not to mention bargaining. "Any car that's been on the lot for a long time loses its value in the eyes of the car salesman," he says.

Heading to the dealership on a weekday morning also helps because there's low foot traffic, meaning you'll have ample time to negotiate and fewer people trying to buy the same car. The more demand, the less willing a salesman is to go down on price, says Reed. (For more, see our column Summer Car Savings.)

Gas

When to buy: Early morning or late evening on a weekday.
Why: Time your trip based on whether prices are rising or falling, advises Marshall Brain, founder of HowStuffWorks, a consumer guide. Gas stations tend to change their prices between 10 a.m. and noon, so hit the pump in the early morning if gas prices are on the rise. Go later in the day if prices are falling. Tipsters on GasPriceWatch.com reported that on Sept. 3, a WaWa gas station in Lanoka Harbor, N.J., was offering regular gas for $2.85 a gallon. One day later the station's price had dropped to $2.65. In that case, going early would have cost you 20 cents more per gallon.

Try not to buy gas on the weekends, Brain says. Gas prices are often slightly elevated, as stations try to profit from leisure travelers. (For more ways to save, see our column Save on Gas.)

Groceries

When to buy: Sunday evenings.
Why: Store sales tend to run Wednesday through Tuesday, says Teri Gault, founder of The Grocery Game, a consumer savings program. On Sunday, you'll also have the latest round of manufacturer's coupons from your morning paper. "You can maximize your coupons available for that shopping week," she says. Heading to the store close to closing time means you'll have access to sales on fresh items that must be sold by the end of the day, such as meats and baked goods.

Of course, you'll also benefit from in-season items that can be frozen for use later in the year, says Gault. That means turkeys at Thanksgiving and hams at Christmas and Easter. During the spring and summer, buy fresh produce. Peaches bought at $1 per pound now can be kept frozen for smoothies and pies throughout the winter, she says.