Most of the Muslim world is preparing to mark the start of Islam's holy month of Ramadan.

Islam follows a lunar calendar, so the start of the holiday is determined by the appearance of the new moon. Religious leaders say no moon was sighted Thursday, so Friday has been designated the last day of the month preceding Ramadan.

Muslims in countries including Algeria, Egypt, Indonesia, Iraq, Sudan, Syria, Somalia, the Gulf Arab States and the Palestinian territories will begin fasting at daybreak on Saturday.

During Ramadan, Muslims are expected to abstain from eating, drinking, smoking and sexual relations from dawn until sunset.

The ninth month of the Islamic calendar, Ramadan marks the time more than 1,400 years ago when Muslims believe the words of Islam's holy book, the Koran, were revealed to the Prophet Mohammed.

U.S. President Barack Obama welcomed the beginning of the Islamic holy month in a video address posted on the White House Web site. The president said he wanted to extend his wishes for a blessed month to Muslims in America and around the world.

Ramadan will continue through September, concluding with a celebration called Eid al-Fitr.

Some information for this report was provided by AFP and Reuters.