Volunteers hand out toys at a Mary's Center Christmas party. The non-profit sought to raise $175,00 in the last two weeks of December to meet its annual fundraising goal. (Bill O'Leary/The Washington Post)

The e-mails flooding inboxes practically shout with urgency: “Time is running out!” “Don’t delay, give today!” Or this, from the Heritage Foundation: “Our country needs you today — before ­midnight on Wednesday — like never before.”

What they need, specifically, is your last-minute, tax-deductible donations: Cash, checks, stocks and IRA transfers delivered by 11:59 p.m. on Dec. 31. For professional fundraisers, the end of the calendar year is the final, often frantic push to coax contributions from donors large and small.

These are not the annual pledges most organizations cultivate year-round. These are the last-minute, often impulsive gifts motivated by a burst of seasonal generosity, an unexpected holiday bonus or a savvy financial adviser crunching year-end numbers who says: “You can either give it to the government in the form of taxes, or you can give it away to a charity you care about.”

So while most of Washington shuts down, this is the one time of the year development professionals never take off. Charities are filled with staffers manning phone lines and monitoring e-mails. Office lines are forwarded to personal cellphones so not a single call is missed. Doors remain open after normal hours on New Year’s Eve — often late into the night.

“We definitely take advantage of the time of year because of tax deductions,” says Marie Mattson, the Kennedy Center’s vice president for development. “I wouldn’t call it a scramble. I’d call it an opportunity.”

Like most large institutions, the Kennedy Center has a robust fundraising operation all year long. December is the month to send out reminders, make thank-you calls and take advantage of openings that benefit both the donor and the center.

“We asked one of our trustees if they would be interested in matching our year-end gifts,” Mattson says. The answer was yes — to the tune of $2 for every dollar raised. So far, the appeal has brought in $85,000 in new gifts.

Her staff spent most of their time this month following up on check deliveries, making thank-you calls and answering questions about stock transfers and other financial matters. “We have to be on top of any tax laws that change to our advantage or disadvantage,” she says.

Some of those calls were about the IRA extension: Taxpayers 70  1/2 or older must withdraw a distribution from their retirement accounts by the end of the year. On Dec. 19, President Obama signed an extension for 2014 allowing donors to roll over that amount, tax free, to the charity of their choice — but the transaction must be completed by Dec. 31. Charities big and small jumped on the phones to remind their donors of the win-win option.

Other calls dealt with electronic transfers or checks, which must be mailed and postmarked by New Year’s Eve to be deductible in 2014. Occasionally, a donor will miss the letter carrier and deliver a check in person — and someone from Mattson’s staff will be there to receive it. “That’s a busy day,” she says. “We always have to have coverage.”

Georgetown University’s Steve Link has it slightly easier — he’s not stuck in the office because the campus is closed, but his office phone is forwarded to his cell, and his entire staff monitors e-mails 24/7.

As executive director of gift planning, Link deals primarily with wealthy alumni and donors accustomed to making year-end contributions. “For those who had a good year financially, there’s extra incentive to give,” he says. “For some people, Dec. 31 is a good day to give away stock.”

But stock transfers and the IRA rollover require a lot of paperwork — a risky process to put off until New Year’s Eve. Link spends a lot of time making sure everyone has the forms they need, then verifying the transactions are completed right up to the IRS deadline.

That’s why he always answers his phone this time of year. He’s had more than one donor tell him, “You’re the first institution to call me back, so you’re getting the gift.”

Americans are a generous bunch: Last year we gave a record $416 billion to charitable causes, and we’re on track to give more than $450 billion in 2014, according to the Atlas of Giving, a monthly guide to charitable giving in the United States.

“If you have high-end donors, it’s a great strategy to focus on them in the last two weeks of the year,” says Atlas president Rob Mitchell. But for most organizations, he believes it is a mistake to count so much on holiday giving, contrary to conventional wisdom.

Tax deductions influence wealthy donors, but only 36 percent of the population itemizes taxes. And the rest? Smaller givers are bombarded by appeals from thousands of good causes during the “season of giving” and can only give to a few.

“Some charities spend 80 percent of their fundraising budget in the last three months of the year,” says Mitchell. “That’s not a good use of their resources.”

People, he says, tend to give because they’re asked, whatever time of year. Traditionally, there’s a slight uptick in donations during the last quarter of the year “but not as big as people might think.” Nonprofit groups would get a better return on investment by stepping back from the holiday rush and tailoring appeals to other times of the year when the competition is less intense.

That doesn’t stop most charities — regardless of what they bring in the rest of the year — from leveraging the season for maximum gain. The Community Foundation for the National Capital Region operates a number of individual donor-advised funds and its own Fund for Greater Washington.

“Our donors are giving throughout the year, but from Thanksgiving until December 31 is the busiest for us,” says Angela Jones Hackley, interim president. The foundation will raise 25 to 30 percent of its budget in just five weeks; last year $10 million came in during the month of December.

The art is all in the ask: Too many year-end calls or e-mails can turn donors off. Too few and an opportunity might be missed. Like for Goldilocks, it has to be just right — a delicate balance of appreciation for prior giving, sincere holiday wishes and appeals for one last donation of the year.

Instead of approaching longtime supporters, Studio Theatre focuses on young donors, who tend to be more impulsive and do the bulk of their charitable giving from Thanksgiving to the end of the year. That’s why the theater launched a digital marketing strategy on Black Friday, says managing director Meridith Burkus.

“What we’re really trying to do is capi­tal­ize on their interest in the neighborhood,” she says. “It’s awareness more than, ‘Give now! Give now!’ As it gets closer to the end of the year, we start to emphasize the tax deadline.”

Mary’s Center, which operates four health clinics for low-income residents in Washington, blasts about 10,000 donation requests almost every day up to the new year. In the last two weeks of December, the nonprofit sought to raise $175,000 to meet its annual fundraising goal.

“I will send out an e-mail every day except Christmas,” says Rebecca Diamond, chief development officer. “On Christmas Eve, it’s a ‘Merry Christmas’ message and thank you. On December 26, they’re over the Christmas hump and thinking about their taxes.”

A few $10,000 checks will come in at the last minute, and there’s often a surprise gift of $25,000 or more — and each one is a thrill, Diamond says: “You feel like you’ve worked all year for these 10 days.”