Muslims around the world are marking the start of Islam's holy month of Ramadan.

Some countries began observances Wednesday. But Indonesia, the world's most populous Muslim country, and most Middle Eastern nations have marked Thursday as this year's start date.

The timing of the start of the holiday varies in Muslim communities and countries depending on the sighting of the new moon.

President Bush sent his greetings to Muslims observing Ramadan. He called for the holy days of Ramadan to remind everyone to seek a culture of compassion and serve others in charity.

Ramadan, the ninth month of the Islamic calendar, marks the time 14-hundred years ago when Muslims believe the words of Islam's holy book, the Koran, were revealed to the Prophet Mohammed.

During Ramadan, Muslims fast from sunrise to sundown -- abstaining from eating, drinking, smoking and sexual relations. The fast is broken at sundown each day with a feast called "iftaar."

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