Fifteen months ago, 20 consumer groups and labor activists received a letter from Mark Green, the director of Ralph Nader's Congress Watch project, Green told the recipients of the letter that the business lobby in Washington was doing all too well in challenging the successes of the consumer movement that characterized the early 1970s.

Prompted by Green's letter, many of these activists started meeting in what they came to call the Other Roundtable, a takeoff on the Business Roundtable, which in Washington represents the opposition in many of the battles Green and his coleagues fight.

The result of the Other Roundtable's meetings and months of planning is "Big Business Day," set for Thursday. Big Business Day is designed to focus the groups' attacks on the practices and policies of corporate America.

"The fundamental premise of Big Business Day is that shareholders are just one of many 'stakeholders' of a corporation," Green said. "Yet other than shareholders, these other stakeholders, like workers, consumers and local communities, are denied access to a voice in these giant private governments."

In addition to Green, organizers of the program include economist John Kenneth Gailbraith; William Wynn, President of the United Food and Commercial Workers; Douglas Fraser, president of the United Auto Workers; Patsy Mink, president of the Americans for Democratic Action, and James Farmer, president of the Coalition of American Public Employees. These organizers and others will participate in rallies across the country.

Is Big Business Day a one-day shot designed simply to get under the skin of corporate executives, or is it more? According to Green and other organizers of the project, the legacy of their activities could take years to measure.

If the goal of the effort was simply to anger corporate executives, Big Business Day already can be considered a success. "The nation's real problems relate to inflation, energy, productivity and national security, and it is unlikely the public will have much sympathy for this ideological Woodstock on April 17," said Irving Shapiro, chairman of E. I. du Pont de Nemours & co., one of the program's targeted firms.

A barrage of counter literature and programs have been distributed by advocates of Growth Day, scheduled for the same day, when business representatives and some economists will compete with Big Business Day for public attention.

But according to Green and other organizers, their efforts will not end at midnight April 17. "We intend to have an ongoing organization emerge from that day," Green said recently. "The day is not the culmination but the kickoff of ongoing citizen oversight of big business."

Secondly, the coalition is pushing for congressional action on the Corporate Democracy Act, a bill that advocates hope will make corporations more accountable to shareholders and the communities they serve.