Opening night of the first pre-Broadway run, at the Forrest Theatre in Philadelphia, was February 24, 1976. There were to have been two previews before opening night in Philadelphia, but the first preview, scheduled for Feb. 21 (a Saturday evening), was canceled. The first (and only) preview in Philadelphia was on February 23.
According to some online sources, the date of the first Broadway preview was April 21, 1976. That is incorrect. The first Broadway preview was on April 22.
Rehearsals started January 20, 1976.
The second of the two tryout runs was at the National Theatre in Washington, D.C. Opening night there was originally scheduled for March 17, 1976. The opening was delayed till March 24, while changes were made. Preview performances were given during that time.
It is hard to be sure at this point when previews started in Washington. The website for the National Theatre gives the date of the first performance there as having been Monday, March 15, suggesting that two or three previews may have been planned before the originally scheduled opening date on Wednesday of that week. It's possible that the start of previews there was delayed. Two previews had been scheduled for the first tryout run, at the Forrest in Philadelphia, but the first of those two previews was canceled, so it's quite possible that the start of previews in Washington was also delayed, especially given the major changes being made to the production.
During the tryout run in Philadelphia, director Frank Corsaro and choreographer Donald McKayle both left the production. According to Erik Haagensen's article on the show, which was published in the Fall 1992 issue of Show Music magazine, Corsaro asked to be fired. It may well be that McKayle's departure was also voluntary.
Corsaro was replaced by Gilbert Moses, while McKayle was replaced by George Faison. Their billing, however, was perhaps a bit confusing and not altogether accurate in that they were jointly credited with the show's direction, staging and choreography. It read "Entire Production Co-Directed, Staged and Choreographed by Gilbert Moses and George Faison."
Despite the huge changes and turmoil that went on between the first performances in Philadelphia and opening night on Broadway two-and-a-half months later, the production opened on Broadway on May 4, 1976, the date that had been announced before rehearsals started.
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