Handbook of Liturgical Practice of the Russian Church Abroad
Clergy who have the right to wear head coverings in the temple, whether by obedience (e.g. the klobuk of an Hieromonk or black kamilavka of an Hierodeacon) or by ecclesiastical award (e.g. the purple skufia or kamilavka of a Priest, the purple kamilavka of a Protodeacon), must wear these during the entrance prayers before the Divine Liturgy. These are not to be set aside out of improper informality.
The head coverings are worn from the beginning of the entrance prayers and throughout. They are temporarily removed for the veneration of the icons of Christ and the Theotokos at the end of the prayers (as we never venerate the holy icons with our heads covered), and remain off during the prayer of bended neck (‘O Lord, stretch forth Thy hand from Thy holy dwelling place on high…’). They are returned to the head at the conclusion of the prayer, before the clergy turn to bow to the people whilst asking their forgiveness; and they are worn as the clergy enter into the Altar for the conclusion of the prayers there. They are removed prior to venerating the Holy Table and the Cross, and are then set aside whilst the clergy vest, to be returned again to their heads before the beginning of the Divine Liturgy.