These are the days of the calling of the roll and the changing of the guard in Washington. As such, it is a familiar -- and reassuring -- American political ritual, one that has occurred without break for nearly 200 years whether in good times or bad, war or peace, boom or bust.

Yet this time the change involving the assumption of powers of a newly elected Congress and a newly sworn president takes place in an atmosphere charged with an unusual sense of history and high stakes:

We are about to witness the 50th inauguration of an American president. Halfway into his term, the nation will mark the 200th anniversary of the adoption of its Constitution (and thus in a real sense the true bicentennial of the American governmental system). At that same time, the 100th Congress of the United States will have just taken its seat.

Fate, luck, history and the will of the people have conspired to put the oldest man ever to serve as president into the White House during this period.

Two weeks after he takes his inaugural oath at the Capitol, Ronald Reagan will celebrate his 74th birthday. Consider how extraordinary that fact alone is:

Among notable, if not great presidents, Washington and Jefferson were 57 when first inaugurated, Lincoln 52, Wilson 56, Franklin Roosevelt 51 (and, after being elected four times, only 63 when he died), Eisenhower 62 (and, after his heart attack and major intestinal surgery while in office, had just had his 70th birthday when he left office), Kennedy 43.

Reagan, with continued good health and luck, will be the first president to serve a full two terms since Eisenhower a quarter century ago -- and the only one ever to have been in power in his 70s. Nearly half of America's citizens have never known a time when a president remained in office for eight years.

These are unusual elements enough to set apart the 1985 ceremonies at the Capitol. But there are others more significant than a recitation of historical facts, figures and precedents.

In the past few days, much has been made of the potential impact of the departure of so many top-level White House aides simultaneously with the beginning of Reagan's second term. Three of those leaving -- William P. Clark, Michael K. Deaver and Edwin Meese III (though he is staying in government) -- have been intimate aides who have served him loyally and well since his California governorship days.

Reagan, far more than most presidents, has been removed from the process of governmental decision-making in the White House. Even his most ardent admirers have never portrayed him as particularly well-informed and, on occasion, he publicly displayed an appalling lack of information and knowledge.

Again more than most presidents, he has delegated his authority and relied on the advice and skills of such key and close aides. They have therefore been all the more crucial to his success. Thus, the legitimate speculation about what these departures mean. It would not be a surprise, either, if others around him are gone by spring.

But these are by no means the only major changes that have taken place in his presidency. The turnover rate in his administration already has been great.

Of those who came to power with him four years ago, think how many have been long gone, whatever the reason. They include:

His original domestic policy chief, Martin Anderson; his political aide, Lyn Nofziger; his national security affairs adviser, Richard V. Allen; his secretary of state, Alexander M. Haig Jr.; his secretary of the interior, James G. Watt; his Environmental Protection Agency head, Anne Burford Gorsuch; his secretary of transporation, Drew Lewis; his secretary of health and human services, Richard S. Schweiker; his secretary of education, Terrel H. Bell; his press secretary, James S. Brady, his director of communications, David R. Gergen.

There are others whose names do not come to mind. Of some, such as his embattled secretary of labor, Raymond J. Donovan, it would not be inaccurate to say that they never should have been there in the first place.

None of this would be important if the next four years shaped up as easy ones for political leaders, a placid time such at the Coolidge years 60 years ago when things took care of themselves -- or seemed to -- and no hard decisions had to be made. They promise to be anything but.

This new Congress and this new administration face a series of challenges of a magnitude and complexity perhaps unmatched since World War II, and at a time when the reality is that American government has not been functioning well.

For years now, and dating from the bitterness engendered by the Vietnam war, a sort of stalemate has existed in the relations between the White House and Capitol Hill. Action occurs only when a crisis exists, and a crisis that no longer can be postponed, such as the Nixon impeachment proceedings or the looming bankruptcy of the Social Security system. Often the measures taken tend to be short-term with the tougher, longer-term decisions postponed.

Which is exactly what has been happening for two years on the budget deficit, and trade deficit, too, for they are in fundamental ways inseparable. In this area, as in arms control negotiations with the Soviet Union, the next four years are fateful. A president and a Congress that fail to deal with them will leave the world in a more fragile and dangerous state.

So enjoy the present lull and round of parties in Washington, new members of Congress, and rest well while you can, Mr. President. The buck really does finally stop somewhere, and like it or not, this time it stops with you.