Tehran Bureau puts together a primer on the historical significance of today's date - known on the Iranian calendar as "13 Aban":

Nov. 4 marks the the day, 30 years ago, when Islamic leftist students stormed the U.S. Embassy in Tehran and took 69 people hostage. Seventeen of them were released at various times, but the remaining 52 remained captive for 444 days. The event was a watershed in U.S.-Iran relations with repercussions still felt today. The main consequence of that event has been that the United States, the most powerful nation on earth, and Iran, the most important country in the Middle East, have not had diplomatic relations for three decades.

Nov. 4 also marks the anniversary of two other important events in the history of contemporary Iran. On that day in 1964, Shah Mohammad Reza Pahlavi forced Ayatollah Ruhollah Khomeini into exile, first to Turkey and then to Najaf, Iraq. The Ayatollah returned to Iran triumphantly on February 1, 1979, after the Shah had gone into exile on January 16, 1979.

It was also on Nov. 4, 1978, that a gathering of students on the campus of the University of Tehran was attacked by the Shah's security forces. Scores of young people were killed, including students as young as 13. That event made it clear that the confrontation between the Shah and the Iranian people had entered its final stage, and that it could end only if the Shah was removed from power.

Much more here.