FOR THE FIRST TIME in seven years, the middle-class residents of Managua's La Reforma neighborhood gathered in the park in front of their houses last week to celebrate Nicaragua's most important religious holiday, the Immaculate Conception of Mary -- known here as "La Purissima."

"After the (1972) earthquake, we all moved away until our houses were rebuilt," said Irma Gonzalez.

"Then came the war, and no one really felt like celebrating."

But this year, all over Nicaragua, homemade altars have been set up in homes, offices and parks and the feast of the Virgin Mary is being celebrated with what one Nicaraguan journalist called "a very special spirit."

In La Reforma, the neighbors carried folding chairs out to the park and set them up around the altar, draped with blue and white crepe paper and topped with a statue of the Virgin and a tinfoil crescent moon studded with Christmas tree lights.

As the older boys set off volleys of firecrackers, rockets and roman candles, the men and women sang lively songs praising Mary. Women walked through the crowd giving each person a traditional offering of bananas, sugar cane, candy and -- a modern innovation -- popsicles.

Another innovation in this first Purissima after the Sandinista overthrow of dictator Anastasio Somoza is that many of the religious celebrations, including the one in La Reforma, were sponsored this year by the Sandinista defense committees. The committees, which have been organized in every neighborhood, began as clandestine support for the guerrillas and are now expected to form the base for a political organization.

In La Reforma and many other middle-class neighborhoods, the committee is solidly Catholic while its members clearly support changes in Nicaraguan society, their values and life-style are very different from those of committee members in the slums and villages that suffered most during the fighting.

Political observers here are watching carefully to see what kind of political organization will develop from the very diverse committees.

THE SPIRITED Purissima celebrations are only one sign of the new energy and enthusiasm visible in Managua these days.

For seven years after the 1972 earthquake what had been the center of the city was left to become a stark wasteland. Block after block was covered with weeds three to four feet high, punctuated by an occasional crumbling ruin.

Poor families lived for years in the shells of fallen buildings. The Somoza government, saying it was dangerous to rebuild the capital in an earthquake zone, promised to develop the outer ring of suburbs. But it never bothered to cut the weeds or tear down the ruins, and the "shell of Managua" as it is called, was a permanent visual affront to residents and visitors.

Now in central Managua from first light until well after dark, workers are clearing away the weeds. They are also building foundations and laying bricks.

The project employs 8,000 workers and is a graphic public relations triumph for the new government.

The park and recreation area under construction is to be named for a boy killed by Somoza's National Guard.

Nicaragua's new rulers, who fought long and hard to overthrow Somoza, are determined that the sacrifices of the war will not be forgotten. It seems that every street, park, school and alleyway has been renamed for one of the often cited "heroes and martyrs" of the war.

Roosevelt Avenue, named for the president whose marines trained the hated National Guard, is now Sandino Avenue, named for the Nicaraguan rebel who fought them and inspired the Sandinistas. THE ONLY significant shadow cast on what orators are calling the "first purissima in free Nicaragua" was a government decree cutting everyone's Christmas bonus from one month's pay to a straight 1,500 cordobas, or $150 at the official rate of exchange but only $93 at the more realistic unofficial rate.

The rest of the traditional "thirteenth month" will go into a fund to create jobs for the unemployed.

The announcement caused considerable grumbling among people who had planned to pay their bills with the Christmas bonus and merchants who said it would cut seasonal sales. There were even a few brief wildcat strikes.

Government economists said paying the full bonus would cause an inflationary surge that the country could ill afford.

They added that they did not expect unhappiness with the measure to sap the government's strength.One pointed out that 95 percent of the salaried workers in Nicaragua earn less than 1,500 cordobas a month.