Eid al-Adha, the Muslim festival that marks the end of the pilgrimage to Mecca known as the hajj, will be celebrated on Monday.

The holy day’s observance is set by sighting of the new moon by religious leaders in Saudi Arabia. This year, there was a possibility that it would fall on Sunday.

That prompted concern on the part of some Muslims on Long Island, with Sunday being the 15th anniversary of the Sept. 11, 2001, terrorist attacks on the World Trade Center and the Pentagon.

Others thought it unfair that Muslims even address that issue because many activities by other groups — church and school picnics, for instance — also are scheduled on that day.

“I’m glad it’s happening on the 12th, without any conflict, without any suspicions,” said Nayyar Imam, a leader of the Islamic Association of Long Island, a mosque in Selden.

Eid al-Adha, also known as the Feast of Sacrifice, comes one day after the hajj, during which more than 2 million people are expected to arrive at Mecca. Muslims who are physically and financially able are required to make the pilgrimage once in their lifetimes.

Mecca, considered the holiest city in the religion of Islam, is the birthplace of Muhammad and the site of Muhammad’s first revelation of the Quran.

advertisement | advertise on newsday

Imam and other Muslims said that if Eid al-Adha had fallen on Sept. 11, they would have celebrated the holy day with prayers and family gatherings as they do every year.

“We have to celebrate it,” Imam said. “This is one of the holiest days for the Muslims. It’s just like Easter or Christmas, basically” are for Christians.

Sept. 11 “is a somber day. We know that,” said Habeeb Ahmed, a leader of the Islamic Center of Long Island in Westbury. “But life has to go on.”

Ahmed said he will be taking part in Sept. 11 memorial services, and that he would have done both if Eid al-Adha fell on the same date. The mosque in Westbury is unveiling a plaque on Sunday honoring victims of the terrorist attacks at a “peace garden” it created on its grounds.

Ahmed and others stressed that they do not consider the Sept. 11 attackers to be Muslims, because their religion prohibits violence.

The 19 men who hijacked four aircraft that day 15 years ago were affiliated with al-Qaida, and 15 of them were from Saudi Arabia.

Dr. Yousuf U. Syed, another leader of the Selden mosque, said he will take part in a Sept. 11 memorial event on Sunday at a Presbyterian church in East Hampton, and celebrate Eid al-Adha the following day at the mosque.

The main message he wants to deliver this year, Syed said, is that “we Muslims in the majority are seeking peace.”

Eid al-Adha, the faithful believe, commemorates the biblical patriarch Abraham and his willingness to sacrifice his son, Ishmael, at God’s command. Abraham did not kill his son, as God spared the boy. Instead, Abraham sacrificed a ram.

During the festival, many Muslims have an animal, usually a lamb, slaughtered. They typically distribute one-third of the meat to the poor and another third to relatives and friends. They keep the final third for themselves.

advertisement | advertise on newsday

A local pediatrician, Dr. Hafiz Rehman of Bay Shore, said last week he planned to make the pilgrimage to Mecca that precedes Eid al-Adha.

“It’s the journey of a lifetime,” he said, and is “very spiritual. You are bowing down to the Almighty, your creator. You ask for forgiveness for any sins you may have committed, knowingly or unknowingly.”

“When you come back from the hajj, you should be like a newborn baby, meaning you are clean and clear of all sin,” he added.