Kari Ansari recalls getting ready to celebrate one of the most important religious holidays of the year — the end of the monthlong Ramadan fast.

She and her husband bought new clothes and gifts for their three children and planned a special family meal. But there was one obstacle to starting the celebration: Leaders of the two local mosques couldn't agree when the feast, called Eid al-Fitr, should begin.

"We would just be sitting up at night waiting to hear the decision," said Ansari, who lives in Herndon, Va., and is editor of America's Muslim Family magazine.

The Muslim practice of following a strict lunar calendar, requiring a naked-eye sighting of the new moon to start a holiday the next morning, has divided the Muslim community on its most sacred days. Now a scholarly panel that advises American Muslims on religious law is trying to end the confusion.

The Fiqh Council of North America announced last week that it would no longer rely on moon sightings to determine the start of holidays and would instead use astronomical calculations. The panel released an Islamic calendar that runs through 2011, hoping Muslims in the United States and Canada can be persuaded to trade the old way for the new.

The schedule problem is more than a minor inconvenience. School calendars and vacation time from work, for instance, depend on knowing dates in advance.

"There will be a lot of resentment at first," said Khalid Shaukat, an astronomer and research physicist with the U.S. Nuclear Regulatory Commission, who calculated the calendar for the Fiqh Council. "But I expect that as the time goes on and we educate them, people will see the benefit of this and understand that what may seem like a new approach to them is not against Islamic jurisprudence."

The date of the Eid is based on the Hadith, traditions taken from the life of the Prophet Muhammad. The prophet taught that the holiday marking the end of Ramadan comes the morning after a nighttime sighting of the new moon.

Under the most conservative interpretation, two credible witnesses with expertise in Islamic sharia law have to see the crescent moon with the naked eye before their observations can be accepted, said Sulayman Nyang, an expert on Islam at Howard University.

But the Fiqh Council contends that the prophet used direct sightings only because no other method was reliable in his lifetime.

"Now, we know scientifically whether the moon is there, even if it is not sightable because of the weather conditions," said Muzammil Siddiqi, the council chairman.

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."