Gary,
First, here’s the liturgical norm
58. For the pastoral advantage of the people, it is permissible to observe on the Sundays in Ordinary Time those celebrations that fall during the week and have special appeal to the devotion of the faithful, provided the celebrations take precedence over these Sundays in the Table of Liturgical Days. The Mass for such celebrations may be used at all the Masses at which a congregation is present.
That’s from the
General Norms for the Liturgical Year and the Calendar
ewtn.com/library/CURIA/CDWLITYR.HTM
That document is part of the Roman Missal. It’s liturgical law.
So, Corpus Christi can be observed on a Sunday in Ordinary Time.
What I’m not exactly sure about, is whether this can be done when the Solemnity itself is already a Holy Day of Obligation. The intent behind the norm is that the feast can be celebrated on a Sunday because otherwise, many of the faithful would miss the opportunity to celebrate the feast. However, when Corpus Christi is maintained as a Holy Day of Obligation on its proper day, the presumption is that the faithful do have that opportunity already. I don’t think that #58 was intended to be used to
completely repeat the feast day.
If Corpus Christi (on Thursday) is not a Holy Day of Obligation, then it certainly can be observed in the parish on the following Sunday—that much is clear. Even if he does celebrate it on Sunday, that doesn’t prohibit the priest from also celebrating CC on the proper day, because “it is permissible to observe” is not the same as “it is moved.”
If it’s done as a repeat for the entire parish (again, with Thursday as a day of Obligation)? I don’t see that as being in harmony with the intent of the law, but at the same time, I don’t think it’s outright illicit.
