Jews throughout the world will gather in synagogues at sundown tonight to begin the solemn observance of Yom Kippur, the day of atonement, with prayers and fasting that will last until sundown tomorrow.

This year, the most solemn observance of the Jewish year coincides with the Islamic festival Id al-Adha, the festival of sacrifice, which Muslims everywhere will mark with prayers, feasting and the donation of fresh-killed meat to the poor.

Yom Kippur is the most holy day in the Jewish year, marking the end of the 10-day period of penitence that began with Rosh Hashanah, the Jewish new year. Tonight and throughout tomorrow, observant Jews will attend synagogue services that focus on repentance and atonement for sins committed during the past year and prayers to God for forgiveness.

The Islamic holiday of Id al-Adha traces its origin to the Old Testament story of Abraham. According to the scriptures, Abraham proved his fidelity to God by demonstrating his willingness to sacrifice his son, Isaac. God intervened and Abraham sacrificed a sheep instead.

Id al-Adha traditionally marks the climax of the annual pilgrimage to Mecca. Muslims throughout the world observe it by slaughtering an animal and giving a portion of the meat to the poor.

Three groups of Washington area Muslims will celebrate Id al-Adha Saturday at a site on the Mall, between 14th and 15th streets, directly across from the Washington Monument. Traditional prayers of the day begin at 10 a.m.

Imam Ahmad As-Salam of the Islamic and Universal Cultural Organization said the distribution of meat to the poor will take place "probably between ll and noon."