Whit Monday or Pentecost Monday (also known as Monday of the Holy Spirit) is the holiday celebrated the day after Pentecost, a movable feast in the Christian calendar. It is movable because it is determined by the date of Easter. Whit Monday gets its English name for following "Whitsun", the day that became one of the three baptismal seasons. The origin of the name "Whit Sunday" is generally attributed to the white garments formerly worn by those newly baptized on this feast. In the Eastern Orthodox Church Whit Monday is known as "Monday of the Holy Spirit" or "Day of the Holy Spirit" and is the first day of the afterfeast of Pentecost, being dedicated specifically to the honor of God the Holy Spirit and particularly in commemoration of his descent upon the apostles at Pentecost. The day following is known as Third Day of the Trinity.[5] In the services on the Monday of the Holy Spirit many of the same hymns are sung as on the day of Pentecost itself. During the Divine Liturgy the Deacon intones the same introit as on the day of Pentecost, and the dismissal is the same as on the day of Pentecost. Special canons to the Holy Spirit are chanted at Compline and Matins.

Source:Church Calender