This chapter focuses on “Solidarity Day,” the largest U.S. labor march ever held, and one of the largest marches in all of American history. Held on September 19, 1981, Solidarity Day was organized by the AFL-CIO as a protest against President Reagan’s budget and tax cuts, as well as his claims to speak for working Americans. According to the National Park Service, 260,000 people attended Solidarity Day, more than the number that had turned out for either the iconic March on Washington in 1963 or the Vietnam War Moratorium in 1969. Many reports, moreover, suggested that around 400,000 people had taken part. Closely monitoring events, the Reagan administration estimated that no fewer than 250 organizations had taken part, including 100 unions and a variety of civil rights, religious, and civic groups. Drawing on new archival records and interviews with participants, this chapter provides the first detailed analysis of Solidarity Day. The march showed that Lane Kirkland could fight back and mobilize the AFL-CIO’s mass membership. Sustaining the march’s momentum in the longer-term, however, would be a challenge, especially given the broader conservative political climate.