Historians may look back on yesterday as a watershed, the day the people got a hammerlock on their federal government's spending. Whether or not they do, the nation's capital and its suburbs paid sharp attention to the passing of Sept. 30, 1981.

The biorhythms of this government-dominated region conform roughly to the federal fiscal year, and residents were generally quite aware of the significance that President Reagan intended for yesterday -- that it be the last day of what he says have been years of profligate federal spending.

Washington-area officials and their constituents looked forward to this morning's sunrise with the gamut of feelings from dread to delight: People wanted to know what would happen next.

Harriet Herrman knew more of what to expect than most people, and she knew how she felt. She was livid. Herrman is the director of social services for Montgomery County, and she smiled nervously through clenched teeth yesterday as she helped County Executive Charles Gilchrist describe to reporters the impact of the budget cuts on county welfare programs.

"For 23 years of my life, I've been in this business," she said. "These programs are things I've fought for all my life to get. We finally got them in the '60s, and now they're all out the window." She was trembling, because now it is Herrman who has to dismantle the empire of programs that she helped build.

Ruby Pledgure was frightened. Pledgure, a resident of a District of Columbia senior citizens' apartment complex, feared that she might lose a possession she considers vital: The card that entitles her to Medicaid, the federal medical program for the poor. Pledgure is a diabetic and has medical bills that she couldn't meet alone.

"I can kind of get by on beans and peas, but I can't make it without my Medicaid," she said. "If they take away my Medicaid, I'll die."

James Watkins was apprehensive, but hopeful. For the last seven years, Watkins, a business analyst, has worked for the Community Services Administration (CSA), the final incarnation of the federal antipoverty apparatus set up in Lyndon Johnson's Great Society.

CSA ceases to exist today and Watkins is out of a job. Yesterday, employes held a raucous wake on the street outside the CSA building at 19th and M streets NW, complete with a rock-and-roll band that was cheered when it played the Johnny Paycheck anthem, "Take This Job and Shove It."

"I feel kind of bad, because the communities we've been working to help are going to suffer," said the 44-year-old Watkins, who, nattily dressed and impeccably groomed, seemed representative of the many area blacks who have attained status and salary through federal government jobs, and who now see those attainments threatened.

"It's a loss of income, but I'll make it," he said. "I guess it's a whole new world out there for me."

And on opposite sides of Beauregard Street in Alexandria, people didn't know quite what to think. On one side were the landscaped grounds and pin-neat buildings of The Hermitage, a retirement and nursing home run by the Methodist Church. The Hermitage is already a bastion of the kind of private-sector philanthropy that Reagan predicts will supplement welfare: All patient subsidies come from the church. No Medicaid, no Medicare, no impact from the advent of Oct. 1 and no gloom.

On the other side of the street is the equally well landscaped, equally neat Goodwin House, an Episcopal retirement and nursing home. Goodwin House does use Medicaid and Medicare -- not much, since residents are, in the words of one official, "pretty well off," but enough to make a difference. The facility has already lost a program through which it trained welfare mothers for occupations ranging from nurse's aide to cook.

The face of Reagan's policies is still primarily in shadow. The fear of Ruby Pledgure and the good-natured nonchalance of officials at the Methodist home, are both reactions based more on rumor than fact. In truth, the impact of the cuts is only now being figured out.

Herrman and her staff, and social service agencies in other Maryland jurisdictions, including Prince George's County, are much further along in determining who no longer qualifies for what than either suburban Virginia jurisdictions or the District of Columbia.

Herrman went through the numbers at the news conference with Gilchrist yesterday: About 400 welfare cases dropped entirely, benefits for another 275 families reduced. Her recitation was professional, but the teeth were clenched, the tight smile an exercise in irony.

Finally, Herrman's tone became almost mocking as she described what she believes is a flaw in the Reagan logic. Benefits are being cut to families that have income from sources other than welfare because some member of the family has a job. These are the families that are making it, she said. Taking away their medical benefits, for example, actually provides an incentive for them to quit their jobs, since low-wage employment does not come close to making up for the loss of comprehensive medical care.

With her voice now openly sarcastic, she said she had already heard of one woman who had been working, but who recently quit her job because she was about to lose eligibility for Medicaid, the federal medical program for the poor.

In Virginia, where state officials have reacted with something like glee to most of the cuts, the changes won't take place until Nov. 1. Officials in Arlington County and Alexandria acknowledge that they still haven't been able to sort out the changes and cannot say how many people will be affected.

The good news for residents of the District is that they won't feel the cuts until Jan. 1. The inevitable bad news is that the nation's capital will feel a far greater impact, particularly in welfare, than its suburbs: About 3,500 households eliminated from welfare rolls, another 3,800 with their payments reduced.

Since D.C. officials haven't completed their paperwork, they are preparing to send out notices to all welfare recipients telling them that they may be adversely affected by the cuts. The letters will advise them to answer all future correspondence promptly to guard against their unnecessarily losing benefits.

The letters probably won't soothe many nerves. Apprehension and outright fear have already gripped places like Judiciary House, the apartment complex where Pledgure lives. She may not lose the Medicaid eligibility that she says she can't live without, but she is intensely frightened by what she has heard.

"You've been reading about Ronald Reagan," she said. "He wants to cut it. That's what he said. All the senior citizens are afraid. I asked them if they would all get together so we can talk and let Ronald Reagan hear this. I'm fighting for my Medicaid."

Only one worker was on the job at the United Planning Organization's neighborhood center at Ninth and S streets NW early yesterday morning. Bureaucratic habits die hard: Although the man did not know if he would have a job, or an agency, 24 hours later, he still asked that his name not be used because he is not authorized to talk to the press.

UPO, the city's funnel for antipoverty money, was funded by the now-defunct CSA, and the man did not know whether UPO would exist as the same entity starting today.

"It's pretty shaky," he said. "I don't know whether to come in here tomorrow, or go straight to the unemployment office, or what to do." He also does not know what to tell the agency's clients, whether their benefits will be cut or by how much.

Taped to the window of the office was a hand-lettered poster that read: "Think Positive; Speak Out Now; Stop the Budget Cut; Poor People Are Not The Problem; We Need Volunteers."

The CSA street fair-cum-wake was an upbeat affair, but with an edge of sarcasm and even of bitterness. Employes, wearing T-shirts bearing a caricature of Reagan burying the agency, hugged each other and pledged to keep in touch. One man was seen at an upstairs window guzzling a bottle of wine; when he realized he was being watched, he raised the empty bottle over his head, to the cheers of the crowd.

James Michael Jr. was in the crowd, but he was not a CSA worker. He worked for the General Services Administration, and was helping supervise the logistical aspects of the poverty agency's demise. Cardboard boxes full of records and forms, some destined for use in other agencies and some destined for the National Archives, were piled in the building's entrance.

"Things are going to be very different in the government," Michael said. "People haven't been put through this before. People are out of a job and they don't even know what's happening."

Michael said that his own job is in jeopardy. The branch of GSA where he works is scheduled for extinction a year from now. He looked at the stack of boxes he had helped organize and said, "At least I'll be in good practice."