The Obama administration began telling the vast federal workforce Thursday by e-mail and letter which employees would be required to report during a federal shutdown and which ones should stay home if the government’s doors are closed.

With the prospect of a shutdown looming Saturday, and most employees wondering what to do, officials said those notifications should be completed by Friday.

If there is a shutdown, about 800,000 of the roughly 2.1 million federal workers nationwide are expected to be furloughed, with exceptions for national security and other essential employees.

But uncertainty could linger into next week if a shutdown goes into effect, as some workers report to work to shutter their offices and other agencies are able to stay open for slightly longer periods, officials and workers said.

The notifications came as the government continued to try to explain which federal agencies and entities would and would not be open in the event of a shutdown, and what the overall impact of a shutdown would be.

“It would have very real effects on the services the American people rely on, as well as on the economy as a whole,” Jeff Zients, deputy director for management at the Office of Management and Budget, told reporters at the White House. “It’s a hard, hard, hard situation for people to operate with this level of uncertainty.”

As Washington teemed with spring visitors Thursday and the last weekend of the National Cherry Blossom Festival approached, the prospect of closed museums and offices, idled workers and disappointed tourists grew larger.

And federal workers expressed frustration at the chaos and uncertainty.

“It puts me in a bind,” said George Mitchell, who works in the Department of Health and Human Services, as he stood outside a General Services Administration building on Seventh Street SW.

“Things are tight enough as it is,” he said. “The president has frozen [pay for] us for the next two years. . . . So we can’t get promotions or pay increases for the next two years. Yet still the bills keep coming. The light bill’s got to be paid. The rent’s still got to be paid. . . . It’s tight, and it’s stressful.”

A woman who didn’t want her name used but said she worked for the Agriculture Department said: “I’m in budget, so I know all the confusion it causes.

“We’re just ending the sixth continuing resolution [to fund the government]. We have to do the same kind of work that we have to do for an annual appropriation, over and over again, over and over again. . . . It gets old.”

For Mark Thalacker, who works in the Treasury Department’s Financial Crimes Enforcement office, definitive word came down at a meeting Thursday: His office would be sent home. But his wife, who works as an administrator for a government contractor, was in limbo: She would have to work through the weekend helping to identify which company employees would be furloughed and which would report to work.

“She has no idea” about her own status, he said.

On Thursday night, about 200 anxious government workers and their families packed an emergency town hall meeting in Alexandria sponsored by Rep. James P. Moran Jr. (D-Va.). About 70,000 federal employees work in Moran’s 8th district — the most of any congressional district in the country.

Others are taking to the streets in protest. As many as 500 employees of the State Department and U.S. Agency for International Development are expected to rally Friday at Edward J. Kelly Park on Virginia Avenue NW to show support for colleagues across the government. The rally, organized by the American Foreign Service Association, is expected to draw some lawmakers.

About 85 percent of federal employees live beyond the Beltway. Employees from the departments of Homeland Security, Housing and Urban Development, Labor and Veterans Affairs, among others, rallied Thursday outside a federal building in Chicago. Locally, the American Federation of Government Employees organized a similar rally Wednesday outside the Social Security Administration’s regional offices in Falls Church.

The Pentagon reiterated that national security efforts would continue around the world, and that “military personnel are not subject to furlough and will report for duty as normal during the shutdown.”

Deputy Secretary of Defense William J. Lynn III told Defense Department workers that he would try to keep them updated.

“The DoD will continue to conduct activities in support of our national security, including operations in Iraq, Afghanistan, and Japan,” he wrote in an e-mail, as well as “Libya-related support operations.”

“Inpatient and essential outpatient care in DoD medical treatment facilities” would also continue, along with emergency dental care and dining and child care services.

“Civilians will be briefed by their supervisors by Friday, April 8, as to whether their work and responsibilities fall into excepted or non-excepted status,” he wrote. “Furloughed employees may not telework or volunteer towork.”

Military retiree benefits should continue without interruption, he wrote.

On Capitol Hill, where negotiators Thursday were still trying to reach agreement on a 2011 federal budget, things could, literally, get dirty for two dozen House members who live, as well as work, in the Capitol’s buildings.

If a shutdown happens, their offices would remain open, but the members-only gymnasium in the basement of the Rayburn House Office Building, where they can shower, would be closed, according to House Administration Committee spokeswoman Salley Wood.

U.S. Rep. Hansen Clarke (D-Mich.), a freshman who lives in his office on the third floor of the Longworth House Office Building, would probably use a restroom shower in a tunnel linking two House office buildings or use a different gym, a spokeswoman said.

Downtown, last-ditch efforts continued to try to salvage the annual National Cherry Blossom Festival Parade, scheduled for Saturday. Officials have said the parade would almost certainly have to be canceled in the event of a shutdown.

But some parade participants already were arriving from distant parts of the country. Several hundred youngsters from two high school marching bands arrived Thursday after overnight bus trips — one from Woodland High School in Cartersville, Ga., the other from Fairhope High School in Fairhope, Ala.

Eric Willoughby, Woodland’s director of bands, said his group — 120 youngsters with 20 chaperones and their band instruments — left Cartersville, northwest of Atlanta, at 5 p.m. Wednesday and arrived in Washington at 6 a.m. Thursday.

“A lot of time and effort has been put into it,” he said of the trip. “About $113,000 is what it ultimately cost us to get all these kids here.

“The parade is why we’re coming,” he said. “The disappointment would be pretty intense because that’s what all our preparation has been for. We put in hours of preparation, rehearsing the music.”

If the parade is canceled, “it would be a bummer. It would just be a sour note, sorry for the pun. . . . this is one of the biggest accomplishments our band has had in its history.”

Staff writers Ed O’Keefe, Neil Irwin, Greg Jaffe, Jenna Johnson and Felicia Sonmez contributed to this report.