Handbook of Liturgical Practice of the Russian Church Abroad
The removal of particles from prosfora is an integral part of the Divine Service of the Proskomedia; that is, of the liturgical service of preparation of the holy vessels and offerings for the Liturgy. This means that prosfora offered by the faithful for commemoration in the Liturgy (i.e. with accompanying lists of names of Orthodox living and reposed) are to have particles removed from them only during the service of the Proskomedia, and following its conclusion, up until the time of the Great Entrance. More specifically, particles begin to be removed from offertory prosfora only after the Priest has removed the two main particles for the living, and one for the departed, from the fourth and fifth loaves used in Proskomedia proper; from this point, he may then also remove particles from all prosfora offered by the faithful for commemoration (and other Priests concelebrating with him may from this point — and not before — also make commemorations and remove particles from their prosfora for inclusion on the diskos).
Removing particles from prosfora before the Proskomedia, or after the Great Entrance, is not permitted. While there is sometimes an ‘economia’ granted in extraordinarily large monasteries where many hundreds or even thousands of faithful may come for the Liturgy and therefore offering particles from the prosfora might legitimately require so much time that it may be started the evening before, in our practice this is an exceptional and almost never-employed economia, and is never done without the express blessing of the Bishop. In our parishes, it is not blessed at all. Priests must come to church early enough to perform the Proskomedia piously, with ample time afterwards to make all the necessary commemorations in the proper way — we do not separate this act from its right liturgical place, simply for the sake of ‘convenience’.