The feast had been accepted in 1306 at Cologne; Worms adopted it in 1315; Strasburg in 1316. In England it was introduced from Belgium between 1320 and 1325. In the United States and some other countries the solemnity is held on the Sunday after Trinity.
So, it appears that for a very long time the feast has been observed on Sunday in the United States. Maybe it has something to do with America being a primarily Protestant country vs. a Catholic country?
That could be it. I was not aware that the change had been mandated that far back. I still wish that we would bring it back to Thursday. Of course I would like it to be a Holy Day of Obligation myself in order to bring people back to the Sacrament.
I wonder what the reasoning was for moving it - many other feasts were celebrated on their original days, even though they would have been completely lost on Protestants (Ascension, for example!). That said, I wish we would go back to celebrating all the feast days on their actual days instead of moving them to Sunday, where they lose much of their impact.
That said, at least our priest is planning on having a procession around the plaza after the main Mass on Sunday.
Prior to the Novus Ordo, the Feast of Corpus Christi was universally celebrated on Thursday, even in the US.
However, Pope Leo XIII granted an indult to the US to observe the EXTERNAL SOLEMNITY of the feast on the following Sunday. The term "External Solemnity" has no meaning nor relevance in the Novus Ordo, but is a common occurrence in the Tridentine. Yes, while Corpus Christi can be observed on Sunday in the TLM, the actual feast is still kept today regardless.
The concept of External Solemnity means that 2 Masses of the Feast can be said on Sunday, but the entire Feast and its Office is not transferred and removed from Thursday, as is the case in the Novus Ordo.
Therefore, this coming Sunday is still properly titled the Second Sunday after Pentecost in the TLM. If there are more than 2 Masses in a TLM parish, then there will be Masses for the Sunday, while the High Mass is usually reserved for the External Solemnity.
For the sake of others reading, I reiterate:
An EXTERNAL SOLEMNITY is NOT the same thing as a TRANSFER.
The Feast of Corpus Christi is never and was never Transferred in the TLM; its Mass can be externally solemnized under limited conditions. Prior to V2, today always remained the actual Feast of Corpus Christi in the US and elsewhere, regardless of the allowance for External Solemnity.