Encyclopedia
Liturgy
Liturgical service - the most important public worship of the Orthodox Church, during which the sacrament of Communion is performed. It may be celebrated in a church on one altar only once a day. Outside the church, liturgy may be performed in special cases on altars and portable antimins in specially equipped structures and open areas.
Source: "SP 31-103-99. Buildings, Structures and Complexes of Orthodox Churches"
(adopted by the Decree of the Russian Federal Service for Supervision in Construction dated December 27, 1999 No. 92)