Glenn Kelman, the chief executive of Redfin, an online real estate brokerage based in Seattle that has had a presence on the Island for two years, always thought that houses on the market in the dead of winter were like a “clearance rack.” He was convinced that winter was the worst time to sell.

But instead of relying on “old rules of thumb,” superstition or real estate decisions “driven by emotion,” he said, he surveyed sales of more than 750,000 homes nationwide over a year.

Mr. Kelman’s assumptions turned out to be untrue. Winter, his data showed, is the best time to sell. “People who list in winter have less competition,” he said, citing results released last month that define winter as December through February. “There is such a trickle of inventory that each one gets more attention.”

In a recent telephone interview Mr. Kelman discussed the data, along with numbers from a similar Redfin survey on the most propitious day of the week to list a home — which found that Fridays brought the quickest, most profitable results.

For the best-season survey, Redfin analyzed 753,093 listings that came on the market in 19 metropolitan areas nationwide, for a year starting in mid-November 2009. For the best-day survey, it analyzed 1.209 million listings that came on the market in 17 metropolitan areas from Jan. 1, 2010 to Sept. 1, 2011.

Long Island wasn’t included in the Redfin surveys, but they inspired Jonathan J. Miller, the president of the appraisal firm Miller Samuel, to conduct an informal survey of his own, applying similar search criteria. Perhaps unsurprisingly to some in the field, he found that the Island marches to its own drummer.

Mr. Miller’s results indicated that listing a home on Wednesdays in March results in the quickest sale. His recommendation is based on data crunched from the Multiple Listing Service of Long Island, reviewing closed sales from December 2010 through November 2011.

By this reckoning, Mr. Miller said, the best time to list a home is “at the tail end of winter, just before the spring sales rush.” Locally, listings introduced in March sell the fastest, and March is also the month when most listings are introduced to the market.

Based on the number of contracts signed each month, “ the spring is still the ‘Super Bowl’ of annual real estate sales,” Mr. Miller said. “June is the top month,” he said, for the number of contracts signed, “with March being the month that results in the shortest time on the market.” Homes put on the market in October take the longest to sell.

Grouping the year by day of the week to replicate the second survey, he found that on average, listing a home on a Wednesday results in the fastest selling time — 121 days on the market. Tuesday was a close second, at 126 days. Homes listed on Saturday and Sunday average 131 days on the market. Mr. Miller was also able to deduce that 19 percent of properties are listed on Mondays, with the number of new listings declining through the end of the week, to 7 percent on Sundays.

Regardless of property size or price, he said, “sellers are more optimistic early in the week.”

The Redfin survey indicates that, nationally, would-be sellers holding off for the spring selling season may not be taking the best path. Homes listed in winter sell 1.4 percent closer to their original listing price — $4,900 more, on a $350,000 home.

Even so, 20 percent fewer listings are added in winter than spring. As a result, Mr. Kelman said, “if you have a really pretty house, it gets snapped up right away. They buy it faster; they pay more money.”

When it comes to online house-shopping, he added, January is the month that Redfin enjoys “a massive spike” in Web site activity. “People eat their Christmas ham, pop a cork of Champagne for the New Year, and think about what they are going to change.” When that includes moving, they go online “to see what they can afford.”

Meanwhile, with only one chance to make a great first online impression, sellers try to get an edge. That means decluttering, depersonalizing, doing repairs and staging the property — and then documenting the results with photographs and video tours. For the best shot at selling fast and selling well, said Michael Daly, a Redfin agent for the Island, he posts new listings online on Thursdays and Fridays. That’s when buyers “search for what they are going to tour on the weekend,” he said.

Listing on a Sunday night, Mr. Kelman said, is “coming up a day late and a dollar short.” He added: “You want to be a shiny new toy, top of the toy chest when people decide what houses they want to tour. There is nothing like the spike of being new.” Listings get four times the traffic in their first two days online than in the next two weeks, according to Mr. Kelman.

But Peggy Moriarty, an associate broker with Daniel Gale Sothebys International Realty’s Cold Spring Harbor office, cast doubt on the various findings, pointing out that each area has its own selling patterns. For instance in her area, Laurel Hollow to Lloyd Neck, broker open houses are never scheduled on Fridays. But in Northport, “that’s the No. 1 day to do a broker open house.”

Though more appointments are made as the weekend approaches, “anyone who is a real buyer is on the computer every day,” Ms. Moriarty said. “They want to be the first one in the door.” And of course in the spring market — hers runs from Jan. 15 to May 15 — weather is a factor.

“This business is a roll of the dice,” she said, citing the sellers who, as a “guarantee that their house will sell within a reasonable time,” bury a statue of St. Joseph upside down in the backyard.

Yet common sense still rules. Ms. Moriarty recently timed an open house on a $1.8 million waterfront house in Huntington Bay so that it wasn’t on the day a snowstorm was predicted and also didn’t interfere with the Giants football game.

Dorothy Herman, the chief executive of Prudential Douglas Elliman, expressed similar skepticism about the surveys. “Numbers, you can play and do anything with them,” she said. “If you do an open house on a Sunday, if you have the right price, you’ll sell your house.”