Planning to urge donors to give on the last possible day this year? Plan again.

December 31, traditionally the single biggest day for online giving, falls on a Friday this year. And for many people—especially those who work in government jobs or private businesses—that’s a holiday. Few people will be near their computers to open that last-minute e-mail appeal.

While “last chance” reminders for procrastinating donors might have worked on December 31 last year, because that date fell on a Thursday, the same reminders could put charities’ fund-raising success at risk of not meeting year-end goals in 2010, according to officials at Convio, an Austin, Tex., fund-raising software company.

“We think it introduces some uncertainty,” says Patrick Hansen, an online marketing consultant at Convio. “Start early,” he suggests. “Don’t count on a single e-mail blast on any day, let alone December 31.”

Last year, Convio’s 1,300-plus nonprofit clients reported raising $19-million on December 31, the last day donors could claim a charitable tax deduction. But on that particular day, most people were not taking a long weekend.

Convio’s data show that 35 percent to 42 percent of all online giving occurs in November and December, but December 31 is the “real and tangible deadline” that fund raisers use to create a sense of urgency, Mr. Hansen says. “It’s a psychological deadline.”

This year, however, Mr. Hansen says that charities should conduct their last-minute reminders before New Year’s Eve. Convio’s giving data for 2009 show that the share of people who responded to messages sent on December 28, 29, 30, and 31 were almost the same. This year, “don’t place your eggs in one basket,” Mr. Hansen says. “I would shoot for the 29th. I would shoot for the 30th.”

Other experts like Steve MacLaughlin, director of Internet solutions at Blackbaud, another fund-raising software company, advise charities against concentrating too much on the last day of the year. Instead, says Mr. MacLaughlin, charities should concentrate on the last three months of the year, culminating in December and even in January. “There’s a hangover effect that they should pay attention to,” he says.

Blackbaud data on thousands of client organizations show that it was actually December 28 that was the biggest online-giving day for 2009, not December 31. “It moves around on the calendar,” Mr. MacLaughlin says.

Other fund-raising consultants see little cause for  concern. “My clients are doing a series of messages throughout the month of December, not just the last week” or the last day, says Barb Perell, vice  president of online fund raising at Avalon Consulting, in Washington.

Instead, Ms. Perell says, she is more concerned that charities’ e-mail messages get delivered. She says that she has recommended to clients that they ready their software platforms to be able to handle the heavy volume of e-mail in year-end appeals. “It’s just a very busy time of the year,” Ms.  Perell says.

Some tips:
Send appeals early in the day. Results from past years indicate that the best response to e-mail campaigns occurs in the morning during the last four days of the year, Convio says. Giving drops sharply after lunch hour.
Rework the year-end appeal. Focus on sending a final “last chance” message sometime from December 28 to 30, instead of December 31, especially if many of your organization’s potential donors include people who work for the government or private businesses. Most of them will be off work for the day.
Start in November. Thanksgiving is just around the corner, so plan to make public your year-end online content the week before the holiday. Use direct mail, social media, and other communications techniques so that donors of all types can understand the organization. Charities that schedule e-mail campaigns in advance do better than those that don’t, according to Convio.

To see what one charity has done to prepare a year-end appeal, see this video from Farm Sanctuary, in Watkins Glen, N.Y. And share details with Prospecting readers on what’s working for your group by making a comment to this post.