Divine Liturgy of the Presanctified Gifts, informally called Presanctified Liturgy, is an Eastern Christian liturgical service for the distribution of communion on the weekdays of Great Lent.
The Liturgy is used by..those Eastern Catholic Churches that follow the Byzantine Rite. Great Lent is a season of repentance, fasting, and intensified prayer, and so the Eastern Church regards more frequent reception of communion as especially desirable at that time. However, the full Divine Liturgy has a joyful character not in keeping with the somberness of the season of repentance (Eucharist literally means "thanksgiving"). The Presanctified Liturgy is therefore celebrated instead. Although it is possible to celebrate this service on any weekday (Monday through Friday) of Great Lent, common parish practice is to celebrate it only on Wednesdays, Fridays and those feast days which fall on a weekday.
The Liturgy consists of vespers combined with additional prayers and communion...On the previous Sunday, additional Lambs (Hosts) are prepared and consecrated, enough for all of the Presanctified Liturgies that will be celebrated in the coming week...the consecrated Lambs are reserved, often in a special pyx used only for this purpose.
Then, on the day of the Presanctified Liturgy, during the Kathisma (reading from the Psalter), one of the consecrated Lambs is placed on the diskos (paten) and unconsecrated wine is poured into the chalice. These are then censed and covered with veils. The Great Entrance takes place in a manner similar to that of the Divine Liturgy, except that the priest rather than the deacon carries the diskos, he holds it higher than the chalice, and the entrance is made in absolute silence, while everyone makes a full prostration.
When it comes time for Holy Communion, the clergy make their communion as normal, except that no words accompany their drinking from the chalice. Those portions of the reserved Lamb which will be used to give communion to the faithful are placed in unconsecrated wine in the chalice. - http://en.wikipedia.org/wiki/Liturgy_of_the_Presanctified_Gifts