Union Square was not the same yesterday. The sounds of speechmakers promising a better world through socialism did not rebound off nearby office building and there were no angry banners to undulate in the sun.

Yesterday was May Day and the Communist Party was not there.

"The local committee wasn't on the job" Gus Hall, the party's not-so-old grand old man had said on Saturday. "We have gotten a permit to hold a rally here every year, but you have to get on line for it early in the morning and someone got there earlier."

And so the Communist Party - the party with the name that once paralyzed and terrorized a nation - marked the workers' holiday 24 hours too soon. This had been a tactical lapse, more ironic than damaging.

Gus Hall smiled. It was an easy smile, a shrug-of-the-broad-shoulders smile, a smile that said other slips and other things had hurt much more. This year May Day would be April 30, and if the sun would shine and the crowd would gather, everything would be all right.

The sun did shine and the crowd - about 200 persons - marched and then rallied alt the Square. They wore red flowers pinned to their shirts and they passed out leaflets and sold newspapers and hawked buttons and someone said that in the '30s there had been thousands and someone said that in the '50s there had been none.

"Six-hour days with no cut in pay," they chanted as pigeons scattered and women with babies in carriages and stroilers paused to watch. "End repression in Chile," a poster said. "Free Puerto Rico," demanded another and a local citizen in worn torn clothes left his park-bench home, to join the show.

From the rostrum, the speakers cried for action. A white man spoke. A white woman spoke. A black man spoke. A white girl sang. And then Gus Hall rose to tell the crowd that American capitalism and imperialism were to blame for all their ills and that socialism would one day bring the workers from underfoot and east aside the corporate foe.

When it was over and the Latin band had ceased to play and the people had drifted off. Gus Hall sat in the quiet park behind the Square and talked about his times.

"I think I'm happy," he said. "I have a feeling that it's been a meaningful life and I don't see any other way it could have been as meaningful. It's interesting and in many ways exciting."

It has been a life. Born in 1910 in Minnesota, nine brothers and sisters, his father blacklisted for attempting to organize workers in the local iron mines. By World War II, when he served a hitch as a machinist's mate in the U.S. Navy, he had been a lumberjack, a steelworker, an activist and - starting in 1927 - a member of the Communist Party.

By the late '40s, the "real" trouble began. Communism was the threat and Hall was a leader. Five years in prison at Leavenworth Penitentiary for conspiring to teach and advocate overthrowing the government by force. Three more years for jumping bail and fleeing to Mexico. Then, in 1959 and ever since, the top spot in the party.

"The party went through a 17-year period of tremendous repression under McCarthy and the Smith Act," said Hall, who speaks with the Finnish accent brought over by his parents. "When we won in the Supreme Court and they threw out the anti-Communist laws, we became legal. But we still had problems.

"Anti-Communism remains virulent and we have to constantly struggle with it. People are still afraid because of the McCarthy period, but we have much more room for operation now and are beginning to grow." Gus Hall lives in a kaleidescope. When the late J. Edgar Hoover looked in on it, he described the Communist leader as a "powerful, deceitful, dangerous foe of the United States . . . a fanatical practitioner of the Karl Marx tenet that the end justifies the means . . . a coldly calculating Communist conniver."

Many once agreed and many continued to fear him. Others (he claims 150,000 followers) view him not as an orge but as a knight in somewhat tarnished armor: a knight whose windmills are "imperialism," the "greedy giants of corporate power," "racism," "the Carter administration" and a dozen other enemies.

In person, Hall is a powerful-looking man. His smile is facile, his manner deceptively relaxed and his outward confidence as firm as his handshake. Not unexpectedly, he is ready and willing to comment on the political world around him.

"Although May Day is not a tremendous national holiday in the United States the way it is in socialist countries, I think the working class and trade union movements will again adopt it as a class holiday.

"Detente must take place because the only choice is nuclear war and that's no choice . . . the proposal (Secretary of State Cyrus) Vance took to Moscow was an outrageous one. It wasn't only one-sided, it was so unacceptable."

"Before any other party in the world, we publicly took issue with Maoism and said we had serious questions about the direction it was going It was - and is - going in the direction of nationalism which is dangerous because it can overtake and engulf the party."

"I've made a number of trips to the Soviet Union and I must say in all honesty that anti Semitism is not one of the parts of life there . . . that doesn't mean that there aren't leftovers from Czarist Russia when it was a big thing . . . I have yet to see a Jew say he left because of anti-Semitism. Many say they left because of the lack of Jewish cultural facilities but they found even less here than there."

"There's no way that the United States can evade or postpone for too long the general trend of history. The replacement of capitalism with socialism is an inevitable process that is taking place all over the world. There's no question that the United States will become a socialist country. It's as inevitable as the sun rising tomorrow."

For Gus Hall, the inevitablity was certain, even if only 200 believers turned out on Saturday to indicate their agreement.

"Look, I'm not saying all of this will take place in the immediate future," he said, again with a smile. "I don't think we have reached that stage of development, but it is much easier to be a Communist now. There's much greater receptivity and acceptance. We are still harassed by the FBI from time to time and we plan to make that a public issue.

"Remember, thats how McCarthyism started. It began with the harassment of the Communists and when others didn't take issue with that, it spread and spread and spread. We are warning liberals, progressives and radicals generally that they'd better watch out."

The conversation was over now and Gus Hall, a "card-carrying Communist" who conceded with amused embarrassment that he was not carrying his membership card, rose from the bench to head back to his home and wife in the suburb of Yonkers, N.Y.

A short walk away, a petite white woman with steelgray hair stood chatting aimiably with a 20-ish black man dressed in faded jeans and T-shirt.

". . . and get rid of Big Business," the woman said as the man listened intently. "The corporations are wrapping this country around their fingers. They are everywhere."

"Say," the young man interrupted, tapping the woman on the arm. "Want something to eat? There's a McDonald's across the street."

"Okay," his friend replied. "There's always one around when you need it."