Between November 29 and December 27, guests staying at more than a dozen Fairfax County hotels will be hoping for frigid temperatures outside. That’s because when they book a weekend stay, their Saturday room rate will be determined by the noontime temperature on Saturday at Dulles International Airport. With average December temperatures ranging from 39 to 57 degrees, there’s room for plenty of savings.

Here’s how it works: Weatherbug.com will provide hotels with a reading from Dulles airport at noon every Saturday during the promotion. The temperature at that time is how much you’ll pay for the day—if, say, it’s 41 degrees, then you’ll pay $41.

The promotion, organized by Visit Fairfax, is running at 15 area hotels, including the Westin Tysons Corner, Hyatt Fair Lakes, and Sheraton Reston. Guests must stay a minimum of two days—overlapping a weekend—and book at least 72 hours in advance.

The temperature-driven price applies only to the Saturday rate; all other nights will ring up at the hotels’ normal rates. If the thermometer dip below zero, don’t expect the hotel to write you a check—days with temperatures in the negative numbers are free.

Hotels participating in the promotion can be booked here.