The Liturgy of the Presanctified Gifts
One of the characteristics of Great Lent is that during this period the Eucharistic Liturgy can be offered only on Saturdays (The Liturgy of St. John Chrysostom) and Sundays (The Liturgy of Saint Basil the Great).
Every Eucharistic Liturgy is a feast, a reproduction of Pascha celebrated under one of its aspects: a feast of Christ, Theotokos or one of the Saints. Great Lent is a period of repentance and compunction of the heart so it is not proper to celebrate the feast of the saints in the normal fashion and so the Eucharistic Liturgy.
Before the final institution of the current Cycle of Great Lent Services, the Christian faithful had the custom of taking home from Church consecrated Gifts, so they could receive Communion in the days where Liturgy was not performed. This custom was even more widespread among the monks that were retiring from the monasteries in the wilderness during Lent and for some this was their only food.
This constituted the foundation for the institution of the Pre-Sanctified Liturgy, which is a modified solemn Vespers during which, using the Holy Gifts consecrated the Sunday before, the faithful receive Holy Communion during the days of Lent. The service is performed generally in the evening to allow the faithful to maintain the fast during the day.
The Presanctified Liturgy is performed only during the Great Lent; in current parish practice in the Wednesdays and Fridays of the first 6 weeks and Monday through Wednesday of Holy Week.