For Orthodox Christians, the Lenten Fast begins Monday (3/10) or Wednesday (3/12). Monday begins the Fast for Orthodox in the Byzantine tradition, and Wednesday begins the Fast for Orthodox in the Western tradition. (Both traditions fast for 40 days, but different calculations of fasting days result in different starting times.)
In the Orthodox Church, the Lenten Fast does not ask or require Orthodox Christians to “give up something” for Lent. The Fast is not an individual choice or personal practice, but a community discipline. Therefore, the Fast is not simply abstaining from food or disciplining your appetite. Rather, it prescribes the common rule the faithful are to follow as they fast together. Individuals may certainly choose to “give up” additional items during Lent, but such choices should not replace the Church's fast, and should be made in consolation with individual’s spiritual father.
The church fathers assume that the fast is a communal exercise. They also teach, quite consistently and vigorously, that the fast is more than eating less or abstaining from certain foods. One disciplines the body, they teach, so that one may better discipline the soul. One fasts from food so that one may learn to fast from sin. In the Western tradition, this "true fast" or "spiritual fast" is emphasized not only in the excerpts from the church fathers that are read during Matins, but also in many of the daily collects.
Those interested in the Orthodox Lenten Fast as it is maintained in the Western tradition are directed to this resource and this resource.