On each of the last two Fridays, White House and Justice Department officials scrambled around preparing background briefing papers for what they thought would be a grand announcement of the Reagan administration's comprehensive new immigration policy.

The first date was pushed back because it was too difficult to churn out the background packets before the president headed off to the economic summit in Ottawa.

This past Friday's announcement was put off because of "last-minute consultations with Congress," according to the official line, or because the White House wanted to focus all its attention on the tax bill fight, according to the "background" explanations.

The White House press office already had prepared a presidential statement starting with the true, if predictable, line that "our nation is a nation of immigrants."

The eight-page statement, which was made available to The Washington Post, goes on to outline a program of amnesty for millions of illegal immigrants and promised strict enforcement to prevent future illegal flows that tracks accounts recently leaked to the press.

The delays have exasperated administration officials who worked on the plan, as well as the congressional subcommittees that must turn the proposals into legislation.

Sen. Alan K. Simpson (R-Wyo.) and Rep. Romano L. Mazzoli (D-Ky.), chairmen of the immigration subcommittees in each house, plan to hold joint hearing on the proposals Thursday, after several postponements. It now is possible the announcement will be made at the hearing, one administration official said.

The delays also have given interest groups a chance to ferret out the details and respond to news reports of changes in the proposals. A leading Hispanic was quoted the other day, for instance, as saying the plan to require legalized aliens to have worked here 10 years before applying for permanent residence, and eventual citizenship, amounted to a return to serfdom.

"more than any other country, America's greatness has derived from migrations and our capacity to welcome those from other lands," the president's statement said. "But neither the United States, nor any other nation can absorb all those who would come -- whether to flee persecution or to seek a better life."

The statement also calls for stern measures to prevent another flood of illegal entrants like the Cubans and Haitians who sailed to South Florida last year. Boats will be stopped and searched on the high seas, and illegals detained in what administration officials have recognized could be called "concentration camps."

The most controversial areas involve the provisions on amnesty, identity cards to prove future hires are legal, and the size of an experiemental program of Mexican "guest workers."