September 21 was not the day on which Filipinos officially learned that martial law had been declared. It was, however, the day when then-President Ferdinand Marcos signed Proclamation No. 1081, which placed "the entire Philippines as defined in Article I, Section 1 of the Constitution under martial law." The following sites may prove useful for those who wish to know more about this event, especially those who have no idea what it was like because they had not yet been born 34 years ago—like me =)
The Lawphil Project
Read the full text of Proclamation No. 1081, and compare it with current President Gloria Macapagal-Arroyo's Proclamation No. 1017 or former President Jose Laurel's Proclamation No. 29, which was signed during the Japanese occupation on 21 September 1944.Philippines Free Press
The Free Press was one of the first to be padlocked when martial law was declared. It's interesting to note that as early as 30 January 1971, Napoleon Rama was already asking "Will there be Martial Law?" This magazine declared Marcos its Man of the Year for 1965, and said that the Marcos administration would either be, "a record of futility and ignominious shame, or a testament to Filipino pride and greatness." Its Man of the Year for 1971—before the imposition of martial law—was Benigno "Ninoy" Aquino, Jr., who was chosen because "he stood for the people’s will to resist tyranny, drawing upon himself all the fury of its wrath without flinching..."
Due to the crackdown on local media, only correspondents for foreign publications were able to immediately write about what occurred after the imposition of martial law. If you would like to know how Filipinos reacted—remembering, however, that outsiders looking in are not the same as insiders—read "Marcos' Martial Law" (2 October 1972), "Life in a 'New Society'" (20 November 1972), and "In Search of Normalcy" (1 January 1973).National Security Archive
A National Security Study Memorandum indicates that the US government guessed that Marcos declared martial law due to "uncertainty of being able to remain in power beyond 1973 under normal legal arrangements." The communist insurgency, the memo adds, could have overthrown the government at a future time, but notes that "it clearly does not have that capability at the present time."