It’s a bit difficult to determine when the right time to sit after receiving the Eucharist. Is it when you get to your seat, while the Eucharistic Ministers and the Deacon are cleaning the alter, when the priest sits down, or when the Eucharist is completely inside the Tabernacle?
Each local parish seems to have its own customs in this regard. The General Instruction of the Roman Missal states:
- …as circumstances allow, they may sit or kneel while the period of sacred silence after Communion is observed.
In 2003 the Congregation for Divine Worship and the Discipline of the Sacraments said the follow about GIRM #43:
The *mens *is that that the prescription of the Institutio Generalis Missalis Romani, no. 43, is intended, on one hand, to ensure within broad limits a certain uniformity of posture within the congregation for the various parts of the celebration of the Holy Mass, and on the other, to not regulate posture rigidly in such a way that those who wish to kneel or sit would no longer be free.
Mens means the general thought/mind. Thus the CDWDS is saying that while uniformity of posture is a good thing, individuals after communion are still free to sit or kneel depending upon their personal preference.
You are therefore free to sit the moment you return to your pew or at any time until the congregation stands for the prayer after communion.