Muslim men worship at the Darul Uloom Institute in Pembroke Pines, Fla., on the first day of Ramadan. The lunar calendar that Muslims follow for religious holidays is creating a potential for misunderstandings or worse in a year when American Muslims are already confronting a spike in assaults on their faith and protests against new mosques. Eid al-Fitr, a joyous holiday marking the end of the holy month of Ramadan, this year falls around Sept. 11, 2010. Muslim leaders fear that their gatherings for prayer and festivities could be misinterpreted by those unfamiliar with Islam as a celebration of the 2001 terrorist strikes.

SEATTLE — For Muslims here and around the world, Eid al-Fitr marks the end of the holy month of Ramadan with a day of prayer followed by celebrations.

But in a quirk of the lunar calendar that determines the start of Eid, the holiday this year will occur around Sept. 11, when many Americans will mark with somber remembrance the terrorist attacks that killed more than 3,000 people nine years ago.

This year, prayer services will be held on the first day of Eid, which is Saturday. But to avoid celebrating at a time other Americans are mourning the tragic events, many Muslim families and mosques are delaying Eid celebrations by a day or even a week.

"It's out of respect," said Kabir Jeddy, treasurer of the Muslim Association of the Puget Sound, MAPS, a Redmond mosque. "It's not that we are putting our lives on hold altogether, but it's not a day we want to be out celebrating."

The already sensitive timing of Eid and the 9-11 anniversary has been made even more so by a confluence of developments that in recent weeks and months have put Islam in the spotlight.

A proposed community center and mosque near where the Twin Towers stood before the attacks in Lower Manhattan has divided Americans, with two-thirds of those surveyed in a recent Washington Post-ABC News poll objecting to the proposed complex.

A protest over its siting is scheduled for Saturday in New York, the same day a Florida pastor threatened to burn copies of the Quran, sparking international condemnation.

Ramadan commemorates the seventh-century revelation of the Quran to the Prophet Muhammad, and Muslims mark it in part by fasting each day between dawn and sunset. The timing of Ramadan is determined by moon sightings — with the month beginning with one new moon and ending with the next.

Several prayer services marking the first day of Eid are planned across the Puget Sound region, where the number of Muslims has grown significantly in recent years, with estimates of between 60,000 and 75,000.

Jeddy said more than 10,000 people are expected for prayers at the Washington State Convention Center, and several thousand will gather at the Meydenbauer Center in Bellevue, Wash.

Ordinarily, families and Muslim groups then host Eid parties, or mosques organize larger family style celebrations.

But this year, "I would think most people would have enough common sense not to have parties on Sept. 11 because of its significance for the nation at large," said Imam Humza Chaudhry of the Thawr Institute in North Seattle, a mosque and educational center. "The prayer is the main celebration."

Jeddy said, for example, MAPS' celebration is scheduled for the following Saturday at Remlinger Farms in Carnation, Wash.

Jawad Khaki, president and imam at the Imam Center in Kirkland, Wash., said his mosque this year will forego all celebrations and instead host an interfaith service with a Unitarian Universalist Church.