One day, when their son Samuel is old enough to understand, Cicero Mosqueira dos Reis and Noemi Anaya Lucca will tell him about Thanksgiving week 1987.

They'll describe the pain and the anger they shared. They'll talk about fighting a bureaucracy whose guidelines left no leeway for love.

They'll probably remind Samuel that that was the year Daddy left home.

Mosqueira, 36, a Brazilian laborer who is in this country illegally, plans to leave his family in the District Tuesday under an order from the Immigration and Naturalization Service. His attorney, Felix Toledo, said Mosqueira is qualified to stay in this country but is being forced to leave because the government didn't process his papers in time.

"There's no telling how long they'll be separated," said Toledo, an attorney for Ayuda Inc., an Adams-Morgan organization that offers legal help to Latinos. "The immigration law's central concern has always been family unity and reunification. This situation goes against the spirit of the law."

Robert B. Neptune, director of the Washington district office for the INS, declined to discuss Mosqueira's problems, saying that he is prohibited from commenting on specific cases.

Mosqueira and Anaya, 27, a Puerto Rican, had planned to marry two years ago. But since December 1985, they have been trapped in a legal mine field.

In a final effort to avoid Mosqueira's deportation, Toledo said, the couple applied for a provisional visa for him under an obscure regulation. The rule would allow Anaya as an American citizen to apply for a visa on behalf of her fiance.

It was this application that one INS branch was processing when another branch told Mosqueira he had to leave.

"The law should be flexible, but I understand that with all the different cases, the INS doesn't have time to be flexible," Mosqueira said.

Mosqueira arrived here on a tourist visa in 1984. "I came looking for work," he said. Eleven months later, Mosqueira met Anaya, a teacher, at a party at the Brasil Tropical Restaurant, a gathering spot on Pennsylvania Avenue NW that is popular among Latinos. By then, Mosqueira's visa had expired.

"It was love at first sight," Anaya said.

Mosqueira was equally taken. "I was enchanted by her smile," he said, speaking in Spanish. "There were several others there, but she caught my eye."

The issue of citizenship had not come up, they said. By summer, the couple had planned to marry. Meanwhile, Mosqueira was seeking a divorce from a Brazilian woman.

But soon the INS caught up with him. Mosqueira was taken into custody at the San Juan airport after a visit to Anaya's family. He was jailed for a week before being released pending a hearing.

Events moved rapidly. The INS lost Mosqueira's case file, his lawyer said. On Oct. 12, 1986, Samuel Ernesto Mosqueira was born. Mosqueira's divorce did not become final until the next month. But then, their lawyer told them not to marry because a wedding might run afoul of a new immigration statute.

In November 1986 the new law, known as the Marriage Fraud Prevention Act, took effect. It was designed to prevent Americans from entering into fraudulent marriages to help foreigners gain citizenship. Under the law, a noncitizen facing deportation proceedings who marries an American is barred from the United States for two years.

Toledo then discovered a new strategy. Because Mosqueira and Anaya had known each other for two years, Toledo said, they qualified for a provisional visa. The only requirements, he added, were that Mosqueira had to pick up the visa in Brazil during a quick stopover and agree to marry Anaya within 90 days.

Mosqueira bought a $1,500 round-trip airplane ticket to Brazil, but further legal problems arose. The INS found the paperwork from Mosqueira's arrest a year earlier and began deportation proceedings. Two INS offices were now involved: The visa application was filed with the Eastern Adjudication Center in St. Albans, Vt., and the deportation was handled by a Detention and Deportation Branch office here.

Toledo mailed the visa request to Vermont in June, when proof of Mosqueira's divorce arrived from Brazil. Usually, Neptune said, the turnaround for such an application is 60 to 90 days. So believing the provisional visa would come through any day, Mosqueira agreed to a voluntary departure, Toledo said. He agreed to leave the country at his own expense on a date set by immigration officials: Aug. 11, 1987.

Soon officials in Vermont wrote back, saying the copy of the Brazilian divorce decree wasn't sufficient. They needed the original. They also asked for Anaya's birth certificate.

Toledo complied. Because of the delay, the deportation office granted Mosqueira an extension until Sept. 3.

Enter the Vermont office again. This time, Toledo said, the office returned the application with instructions to wait for further correspondence indicating when it should be resubmitted. The new twist was not explained, the lawyer added. The deporation branch granted Mosqueira a three-month extension.

Then communication ceased.

Toledo said he hasn't heard from the Vermont office since Oct. 22. Toledo sought a third extension on the deportation last week but, he said, the request was turned down.

"It's like someone told you your mom has died," Mosqueira said.

"My heart dropped," said Anaya. "I started to cry."

Thanksgiving 1987 will be remembered for the good as well as the bad, Anaya said. Samuel, active and inquisitive, left them little time to ponder what may happen Tuesday when Mosqueira is set to leave.

But as Mosqueira chased his son around the living room of their apartment at 1514 17th St. NW, he said quietly, "Me siento mal. I feel bad. He just started calling me Papi. If I leave, he'll soon forget that."

Special correspondent Tina Plaza contributed to this report.