Kareem Irfan, of the Council of Islamic Organizations of Greater Chicago, where an estimated 400,000 Muslims live, said the uncertainty of the old system has been costly.

Organizers of the massive community worship services that mark the holiday had to reserve convention halls for two different days, losing money on the double deposit, he said. Muslims who needed a day off from work or had to make plans for pulling their children out of school could not say when the celebration would be.

A patchwork of practices developed, even within the same town.

Some foreign-born imams would follow moon sighting announcements from their native countries. Others followed the decision of the government of Saudi Arabia, where millions of Muslims make pilgrimages each year. It was not unusual to have members of the same family celebrating the holiday on different days.

"It makes you feel sad," Ansari said, "because not everyone is doing the same thing."

The Fiqh Council has spent years trying to end the chaos.

The Islamic Society of North America ran an Eid hot line that took calls from Muslims nationwide who said they had seen the new moon. Once the sun set on the West Coast, scholars and astronomers would hold a conference call, listening as eyewitnesses described what they saw so leaders could decide if the descriptions were credible, Siddiqi said.

The council would then pronounce the start of the holiday, hoping the date would be observed continentwide. "The whole decision would take a long time," said Siddiqi, director of the Islamic Society of Orange County in Garden Grove, Calif. "On the East Coast, it could be 10 o'clock at night, 11 o'clock at night."

The announcement this month about the calendar is the next step in what scholars say will be an intensive effort to win over Muslim communities.

Muslims worldwide disagree about the right method of setting the date.

In Turkey, they use astronomical calculations. The Saudi government has a "double-track approach," Nyang said. Officials there use calculations from an observatory, but decide on a date only after consulting with scholars who follow actual visual sightings, Nyang said.

The first test of the new North American system will come Sept. 23, when, according to the Fiqh Council's Islamic calendar, Ramadan begins.

"The American Muslims aren't going to resolve this problem for the whole Muslim world or even for themselves," Nyang said. "But, gradually, I think science is going to prevail."