The events unfolding in Iran can best be understood in the context of the religious beliefs of the Shiite Moslems who make up more than 90 percent of Iran's population, long-time students of that country maintain.

According to Thomas Ricks, assistant professor of Middle Eastern history at Georgetown University, one of the keys to understanding the situation is the Moslem holy day of Ashura, which began last night and continues until sundown tonight.

A spokeman for Muslim House here said that area Moslems would mark the holy day with a "calm march," beginning this morning. In addition, he said, Moslems of different nationalities would gather to "talk about" Ashura in their native languages at various locations.

The day marks the death by beheading in the year 680 AD of Husayne, on whom the Moslems of what now is Iraq and Iran had pinned their hopes for an Islamic republic.

When the Prophet, Mohammed, died in 632 AD, leadership of the faith he proclaimed went to four successive leaders, known as the "Righteous caliphs."

The fourth caliph -- the father of Husayne -- was Ali. His devoted followers, known as Alids or Shii, believed that Ali's interpretation of the Prophet's teachings was more authentic than those who preceded him and that he should have been the first caliph.

"The Shii argued that Ali's claims [to leadership] were better because of his intimacy with the Prophet and that he had a better idea of what an Islamic state should be, a better understanding of the Prophet," Ricks explained.

When Ali was killed in civil strife in 661-29 years after Mohammed's own death -- his followers were particularly distressed that the fifth caliph, Mu'awiya, moved the capital of the Islamic world from Mecca to Damascus. They were even more distressed to learn at Mu'awiya's death in 680 that he had named his son Yazid as his successor.

The new dynasty "was the last straw to the Shii," who had by that time spread to Iraq, Ricks explained. They asked Ali's son, Husayne, to come to Iraq to establish a rival caliphate, an Islamic republic along more egalitarian lines that they felt fidelity to the Prophet required.

Husayne set out with a large contingent, but was intercepted by Yazid's forces on the plain of Karbala in Iraq. They were held captive for 10 days without food and water. On the 10th day, they were slaughtered by Yazid's forces and the severed head of Husayne was brought back to Damascus.

Throughout Moslem history, the 10 days of the siege traditionally have begun a month of mourning, with the 10th day -- which began last night -- a day of deep mourning, particularly for the Shiites.

Ricks, whose interest in Islam began 15 years ago when he was a Peace Corp volunteer in Iran, pointed out in an interview that the Shiite Moslems of Iran today identify with Husayne, who symbolized for them "the rightful ideals of Islam, which were defeated by elitist forces."

The Iranian Shiites have "an oppressor/oppressed world view," Ricks said. Although they see themselves as the oppressed throughout history, they have maintained the conviction that "the oppressed will one day triumph."

Contemporary Iranian Shiites, he said, see the events of the past year in their country as a fulfillment of that historic yearning: the overthrow of the shah -- the oppressor -- by the oppresed.

"They see the revolution as a vindication of history," Ricks said. He pointed of history," Ricks said. He pointed out that when the shah was overthrown earlier this year, people in the streets of Tehran shouted, "Death to the Yazid of our times."

With the weight of nearly 14 centuries of history upon them, Ricks said, they believe that the oppressor -- the shah -- must be brought to justice.

Since the 17th or 18th century, he said, this beginning of their religious history has been kept alive each year with a kind of Passion Play, called Taziah, Ricks said.

"It is the best known popular theater in Iran," he said. "There are set phrases from it that are known and used by all classes. It is performed [on Ashura] in every town and village of Iran."

The message of that play, he said, is that "someday a man will return who will reverse this [history of oppression]." For Iranian Shiites this year, he continued "that man is [the Ayatollah] Khomeini."

Ricks speculated that Khomeini would "take the occasion to address his people in a very special way."

Ricks, who said he has attended the Shiite passion play "a number of times," says today's holy day traditionally is "a period not of fanaticism, just of heightened sensitivity." CAPTION: Picture, AYATOLLAH KHOMEINI . . . the man to reverse history?