With their observance of Ash Wednesday tomorrow, Christians throughout the world will mark the beginning of Lent. Roman Catholics, high Episcopalians, some Lutherans and other denominations will take part in the Ash Wednesday tradition of receiving ashes on the forehead in the sign of a cross.

The ashes, burned palm leaves from last year's Palm Sunday services, signify penitence and remind the wearer of the period of self-examination.

The 40 days of Lent, leading up to Easter, represent Christ's 40 days of fasting in the desert. For Christians, it is a penitential period of prayer and study. They also are encouraged to perform extra charitable acts during Lent.

Eastern Orthodox churches, which follow the Gregorian calendar, will begin their Lenten observance Monday, which they call Clean Monday. They, too, will mark the period with fasting and introspection.

Several area churches will sponsor special Ash Wednesday services and Lenten lectures. Among them are:

Vienna Baptist Church, Ash Wednesday and Lenten learning center, with films and a puppet show for children, 7:15 p.m., tomorrow, at the church, 541 Marshall Rd., Vienna.

St. John's Church, noon Lenten preaching series, visiting ministers, 12:10 p.m., Wednesdays, beginning tomorrow, at the church, Lafayette Square, 1525 H St. NW.

Twinbrook Baptist Church, Sacred concert by touring choir of Eastern College, St. Davids, Pa., 7:30 p.m., tomorrow, at the church, 1001 Twinbrook Parkway, Rockville.

Church of the Epiphany, Lenten concert series, with organist Dr. Charles Callahan, performing complete organ works of Cesar Franck, 11:30 a.m., followed by traditional Lenten preaching services at noon, every Tuesday in Lent, at the church, 1317 G St. NW.

All Saints Church, family Lenten program with study of Episcopal and Roman Catholic catechisms, family crisis lectures and a children's program 7:30 p.m., Wednesdays (beginning March 7) at the church, Sharon Chapel, 3421 Franconia Rd., Alexandria.

Annual Lenten lecture series, "The Dimension of Hunger," 7:45 p.m., Wednesdays (beginning March 7), at churches in Washington sponsored by the Black Episcopal Clergy Association and the Union of Black Episcopalians, for further information call 547-8400.