The calendar says it's been spring for more than two weeks. The weather in Atlanta, and across the country, insists that spring has been here longer.

But according to Home Depot, spring is just beginning.

The Atlanta home improvement retailer decided months ago when it would begin its spring selling season, which is the company's busiest, Home Depot spokeswoman Jean Niemi said. Planning tools indicated the weather last weekend -- when the third annual Spring Black Friday promotion began -- would be a good one, Niemi said. So spring was born. It started last Thursday.

Across the country, the company promoted weed killers and lawn mowers, patio furniture and grills. While the weather has been unseasonably warm nationwide this year, Niemi said the actual temperature doesn't have a bearing on when people start to dream of landscaping and cookouts.

"Our research shows March is a popular time for lawn and garden planning regardless of the weather," Niemi said. "Customers get really excited."

In part, Niemi said, Home Depot tied its Black Friday promotion to when customer traffic picks up. The company monitors customer habits and the weather, she said, and tries to get them to align for the sale. While the in-store promotion has ended, sales continue online through Sunday.

This is the first year the sale occurred at the same time across the U.S.; last year, there were three different dates depending on the region -- the first, in the warmest climes, started in mid-March. In colder, northern markets, Home Depot's spring didn't begin until the end of the first week of April. (This year, Alaska still gets spring sales later than the rest of the country, Niemi said.)

While there is some risk of getting the timing wrong -- starting sales well before or long after the weather tells shoppers it is spring -- the company has so far done a good job of bringing customers in the door, said Wayne Hood, managing director for equity research at BMO Capital Markets.

The company likely plans the promotion at least six months ahead of the date, Hood said.

Bill Murphey, chief meteorologist and state climatologist with the Georgia Environmental Protection Division, said while he is able to determine generally whether a winter will be mild or harsh, it's difficult to accurately predict what the weather will be -- or when spring temperatures will come -- more than a couple of days in advance.

For Hood, it's easy enough to determine when spring begins: it's when the weather is warm enough to do outdoor projects. For Murphey, the start of spring is much more consistent, regardless of what Home Depot may say.

"Spring is spring to me," he said. "I go strictly by the vernal equinox."