The long-delayed, $621 million Capitol Visitor Center will open Dec. 2, congressional leaders planned to announce Thursday.

The date was chosen for its historical significance.

It was on Dec. 2, 1863 — during the Civil War — that the Statue of Freedom was hoisted into position atop the Capitol dome. A plaster model of the statue, long on display on the lower level of the Russell Senate Office Building, is being dismantled and will be moved to the new center by October.

In a letter to Congress’ four top leaders, Terrie S. Rouse, chief of visitors services for the new center, advised them that the latest plans call for construction work to end by Aug. 31, followed by the issuance of an occupancy permit.

In September, staff will move in and start testing the systems in the 580,000-square-foot underground facility on the Capitol’s east front.

Members of Congress, constituent groups and families will be invited to come through the center before the formal opening as test users, Rouse added. She estimated that as many as 50,000 people could come through the center during the test phase and before the formal opening.

The center’s costs escalated throughout years of planning and actual construction as the project expanded in scope, a host of construction obstacles were encountered and various congressional committees insisted that top-quality materials be used in a building that is expected to draw hundreds of thousands of tourists from around the world annually.

The ceremonial ground-breaking for the center was in 2000.

Initially, an opening date was envisioned for December 2005. Even the Dec. 2 date represents a delay over the latest estimated opening date of early November.