This was getting-ready week at the Reagan White House in Washington, getting ready for Mr. Reagan's very first excursion into that, for him, unknown world of communism.
He is going to China, not to suitably capitalistic Taiwan China, but to avowedly, overtly communist mainland China.
Enormous preparations have already been made. The advance party has been in Peking, horrifying the Chinese by the number of Secret Service and staff people who accompany an American president on a formal procession in a foreign country, not to mention the horde of reporters and television crews expecting quarters beyond the capacity of China to provide.
The working diplomatic staff is polishing the drafts of various announcements that will be made at the end of the visit. All have been prenegotiated. Together they will make up a communique on the visit. It will appear that diplomatic business has been done.
But the fact of the visit is more important than bits and pieces of diplomatic business that nominally will have been transacted there.
As President, Mr. Reagan has been to Venice and Versailles, to London, Paris, and Bonn, to Seoul, and to Tokyo. But not until he lands in Peking on April 26 will he have set foot on communist soil. In cultural terms, Mexico City is probably the farthest he has yet gone beyond his normal California habitat.
That he is going to China exposes an acceptance by him of a difference between communism and the range of Soviet influence. He continues to talk about the menace he is fighting in Central America and other places as ''communism.'' But he cannot go to Peking without recognizing by so doing that Soviet power does not extend to all places that practice some form or another of Marxism. So the trip marks the acceptance by Ronald Reagan that the communist world is no longer monolithic and that useful business can be done with some communist countries.
It accepts also the fact that China, although communist, has more in common with the United States than it does with the Soviet Union. No top official of China has visited Moscow and no Soviet leader has visited Peking since 1969, when Chou En-lai and Alexei Kosygin met in Peking.
The visit is Part 2 in what diplomats refer to as the ''revival of momentum'' in the US relationship with China.
That relationship began in 1972 when Richard Nixon startled the world by going to Peking. It was developed by President Gerald Ford. He went to Peking in 1975.
It was carried forward another step by President Jimmy Carter, who formalized the new relationship by elevating the diplomatic missions of the two to full embassies and breaking off formal diplomatic relations with Taiwan.
Ronald Reagan campaigned in 1980 for a return to what was called a ''two-China policy.'' George Bush, who was Mr. Reagan's running mate at the time , was sent to Peking to reassure the Chinese government that Mr. Reagan did not mean to break relations with it, but only to retain informal relations with Taiwan.
But the implications worried the Chinese. The relationship chilled when Mr. Reagan became president in 1981.
The revival of forward momentum began in 1983 and was formalized in January of this year when Chinese Prime Minister Zhao Ziyang visited Washington. He asserted afterward that US relations with China had been improved during the visit. The Chinese officially called it ''a success.''
But the chill of 1981 was followed by a change in the Chinese posture toward Moscow. Until Mr. Reagan's election, the Chinese called the Soviet Union ''the most dangerous source of a new world war.'' After his election they agreed to reopen talks with Moscow.
The announcement was made on Oct. 17, 1982. Since then there has been a series of four rounds of Sino-Soviet meetings, back and forth between Moscow and Peking. They were held in March, September, and October of 1983. The fourth round was in Moscow last month, from March 12 to 26.
Beginning with the announcement of the new Soviet talks, the Chinese have tended to criticize the Soviet Union and the US more or less evenhandedly. However, there is a full, official Chinese ambassador in Washington. There is no permanent Chinese diplomatic mission yet in Moscow.
The pending questions in US-China relations mostly concern the terms of trade. The Chinese want to sell more of their textiles in the US market and obtain in return technical help toward modernization, particularly in nuclear matters.
President Reagan's conversion to the idea of improving relations with China came from both State and Defense Departments.
The Defense Department is particularly interested in avoiding a revival of the Chinese alliance with Moscow which lasted from 1950 to 1958.
At present the estrangement between the two biggest communist countries keeps 56 of the best Soviet divisions on the Chinese frontier and safely away from any other part of the world.
Domestic US politics is also involved in the trip. It gives Mr. Reagan a marvelous opportunity to be seen conducting foreign relations in a dramatic setting.
The most interesting speculation among diplomats is whether Mr. Reagan will emerge from his visit to China with some modification of his attitude toward Marxist countries. So far he tends in his policy toward Central America to assume that any country that goes Marxist will automatically move into the Soviet sphere of influence and become hostile to the US.
There has been no hint in Washington of late of any abatement of that assumption in his mind when he thinks about Central America. He is pushing ahead with his effort to undermine the Marxist-led government in Nicaragua in spite of congressional unhappiness about his methods.