His month-long vacation over, President Reagan will find out first-hand on Thursday what union members think of his decision to fire the striking air traffic controllers when he speaks to a gathering here of union carpenters.

Reagan's speech at the centennial celebration of the United Brotherhood of Carpenters and Joiners of America was scheduled before air traffic controllers went on strike Aug. 3, and although White House officials say they believe the president's reception will be friendly, they acknowledge that some members of the audience may be hostile.

Reagan may have received advance indication of what his reception will be when he was greeted here today by two sets of protesters. One group of about 100 gathered at Meigs Field, where the president landed, with signs supporting the fired air traffic controllers. At the Palmer House hotel, where Reagan spent the night, 150 pickets protested the administration's budget cuts, its role in Central America and Reagan's opposition to the Equal Rights Amendment.

The carpenters union officially supports the striking controllers, and Alvin Silverman, a union spokesman, said that a majority of the 2,700 delegates to the convention drove or took trains rather than fly during the strike.

In an Aug. 20 telegram to Reagan, union president William Conya noted the "disastrous situation" created by the president's decision to fire the controllers and rebuild the air traffic control system with new workers. "The net effect of the position taken by your administration is the apparent loss by thousands of hard-working Americans of their jobs and careers, their livelihood and their hope for the future," Conya said.

Conya, who had earlier termed the government's offer to the air controllers a "yellow dog contract" like those industry forced on workers in the early part of the century, urged Reagan to resume the collective bargaining process.

But White House officials continue to say there is no possibility of that. "I think we've gone beyond the point of having to defend what we've done," one said this week.

The White House, as has become its custom recently, released excerpts of the president's speech in advance of its presentation. In those excerpts, he makes few direct references to the controllers' strike. At one point, he says that "from the very first, organized labor predicated its help and support on the condition that public employes could never be allowed to strike. Indeed, they insisted that unions of government employes should recognize this in their constitutions."

Reagan says later in the White House-approved excerpts, "We have the means to change laws we find unjust or onerous. We cannot as citizens pick and choose the laws we will or will not obey."

In other excerpts, Reagan attempts to assure union members that his tax and budget package will restore economic strength in this country.

Describing his economic program, he says, "The foundation has been laid for an American renaissance which will astound the world -- a new era of good feeling in America, a time when jobs will be plentiful and the richness of the country can be shared by anyone who is willing to work."

The president's resolve not to reopen the issue of the controllers was indicated earlier this week when a White House official showed several reporters notations by Reagan to suggestions from members of Congress that he settle the strike. In one case, he scrawled "no way" next to the summary of the congressman's letter; in another case, he simply wrote "no."

Reagan flew from his ranch in Santa Barbara, Calif., to Los Angeles on Sunday afternoon. According to the White House press office, he did not leave his suite during his 2 1/2 days there.

At a fund-raiser at a Chicago hotel tonight, Reagan asked for more time for his economic program to take effect before it is criticized as a failure. Noting the slump in the stock market, he said: "They say, 'Well, the slump is because the program isn't working,' " the president said. "Well, it isn't. It doesn't start until Oct. 1."

He said that as long as inflation continues and "there is a belief that there are going to be huge deficits, interest rates are going to stay up." And he promised to continue to "whittle" at those deficits.

"When you hear the screams of anguish from some whose toes are being stepped on, just think that they can't be half as bad as the moans of the unemployed in this country today, and we're going to put them back to work," he said.

Staff researcher Valarie Thomas contributed to this report.