122 Still sometimes there may be a reason, and that not infrequently, why holy communion should be distributed before or after Mass and even immediately after the priest receives the sacred species - and even though hosts consecrated at a previous Mass should be used. In these circumstances - as we have stated above - the people duly take part in the eucharistic sacrifice and not seldom they can in this way more conveniently receive holy communion. Still, though the Church with the kind heart of a mother strives to meet the spiritual needs of her children, they, for their part, should not readily neglect the directions of the liturgy and, as often as there is no reasonable difficulty, should aim that all their actions at the altar manifest more clearly the living unity of the Mystical Body.
Source: Mediator Dei (Vatican.va)