This feast is celebrated in the Latin Church on the Thursday after Trinity Sunday to solemnly commemorate the institution of the Holy Eucharist.

Of Maundy Thursday, which commemorates this great event, mention is made as Natalis Calicis (Birth of the Chalice ) in the Calendar of Polemius (448) for the 24th of March, the 25th of March being in some places considered as the day of the death of Christ. This day, however, was in Holy Week, a season of sadness, during which the minds of the faithful are expected to be occupied with thoughts of the Lord's Passion . Moreover, so many other functions took place on this day that the principal event was almost lost sight of. This is mentioned as the chief reason for the introduction of the new feast, in the Bull "Transiturus."

https://www.catholic.org/encyclopedia/view.php?id=3400