On February 5, 1977, Marcos decided that the plebiscite would be held after the talk on February 5 – March 3. The plebiscite questions would be limited to the issue as to which of the affected provinces would want to be included in the autonomous region and will not include other issues.
After a week the President decided that the plebiscite would be held on February 21. He directed the Department of Public Information Information, the Department of Local Government and Community Development and the Commission on Elections to conduct intensive information drives to explain the concept and implication of autonomy.
On February 9, the same day that President Marcos signed the decree authorizing the holding of the plebiscite on 21 February, he explained that The decision was the result of the ongoing negotiations in Tripoli.
Two days later, the President changed his mind. He postponed the plebiscite to 17 March “to give our people ample time to learn the exact nature of the Autonomous region.”
On March 17, the President rescheduled the plebiscite to April 17.

Source: B.R. Rodil Kalinaw Mindanao; The Story of the GRP-MNLF Peace Process, 1975-1996