An arbitrary and capricious government says this isn't Memorial Day weekend. But tradition says it ought to be, and that's reason enough for a brief report on a little noted and lightly attended memorial service that took place in Constitution Gardens close by the Lincoln Memorial a week ago.

It added something of consequence to the conventional wisdom about what is loosely called the mood of America, in general, and about public attitudes toward war and peace (the so-called Vietnam Syndrome), in particular.

The setting was a two-acre patch of lush lawn and tall shade trees set aside by a unanimous vote of both houses of Congress as the site of a Vietnam war memorial. A starkly eloquent, "V-shaped stretch of black marble, half-sunk in the earth and bearing the names of each of the 57,661 Vietnam War dead and the approximately 2,500 still missing, has already been chosen as the winning design.

Apart from the deed of federal parkland, this is a strictly private enterprise. Construction of the memorial will be paid for by public contributions a a Vietnam Veteans Memorial Fund. A nationwide fund-raising effort is still well short of its $5 million to $7 million goal.

But there is a message in the $1.4 million already raised and in the astonishing progress made on a project that ws launched almost single-handedly. aIt began when Jan Scruggs, an enlisted man in the Army who saw combat in an infantry unit that suffered heavy casualties, walked unannounced into the office of Sen. Charles Mathias (R-Md.) two years ago.

Scruggs had thought long and hard about some way to memorialize the Vietnam war dead. Mathias' response ws very nearly instantaneous. Within days he had lined up Sens. George McGovern and Barry Goldwater as co-sponsors of the necessary legislation. By the end, all 100 Senate members had not only voted for it, but co-sponsored it as well.

That, in itself, told you something about evolving American attitudes toward this country's least popular, most controversail war. The simple ceremony on the Mall a week ago told more. The managers of the memorial fund were holding their second annual commemorative services. Measured by the number of those who passed up the beach or the Indianapolis 500 or their own backyards on the sun-baked day to be on hand, it was not a particularly impressive event.

There were symbols: a half-dozen gold-star mothers in crisp white uniforms; a couple of bearded veterans in camouflage combat fatigues. But only the bandstand was solidly packed--with the scarlet full-dres uniforms of the U.S. Marine Band. Not many more than 50 of the 500 or so chairs set up in front of the podium were filled.

But the turnout was no measure of the simple, understated force of the event. There was something in the spirit of those on hand--and in their words--that said a lot for the recuperative powers and enduring sensibility of a nation that only a decade ago was caught up in a raging, embittering debate over its first lost war. The point is that the spirit was so low-key.

Who would have expected 10 years ago, for example, that a principal speaker at a gathering of Vietnam veterans would be Ellsworth Bunker, the former ambassador to (among other countries) South Vietnam.

Lean and erect in his late 80s, and economical with words as a Vermont man should be, Bunker is the very model of a model Establishmentarian. And Vietnam veterans, of course, are supposed to be unregenerately turned off by the Establishment.

But there was Bunker, unwavering in his support for the war's aims, while sensitive to its anguishing complexity--"a war new to the American experience . . . conventional and guerrilla . . . political and psychological . . . limited by restraints we imposed upon ourselves. . . . We went in not for greed but to help resist aggression and we brought our adversary to favorable terms."

There was no argument--only remembrance and talk of reconciliation. "This is a place where we will always be able to express love," said one veteran, an organizer of the affair. "They were young when we were young and each one of their names matters." At the end, family and friends filed slowly by a microphone to speak, very simply, a name.

You could make too much of it; there is plenty of anger left, and bitterness against the government. But what was said and not said at the National Vietnam Veterans Memorial Day Service on the Mall speaks well of how, on balance, this country feels about itself and its place in the world a decade after the national trauma of Vietnam.