"Thursday week after Pentecost (Whitsun)" is not wrong; Corpus Christi is celebrated on the Thursday after Trinity Sunday, which is the Sunday after Pentecost. Alternatively, it can be (and customarily is, in some countries including Spain and the USA) celebrated on the following Sunday, "when the faithful are more likely to attend Mass and be able to celebrate the feast."
According to Wikipedia, "The feast of Corpus Christi was proposed by Thomas Aquinas [c. 1225–1274, as] a feast focused solely on the Holy Eucharist, emphasizing the joy of the Eucharist being the Body and Blood, Soul and Divinity of Jesus Christ."
The Eucharistic Miracles website, compiled by the Blessed Carlo Acutis (see below), puts it slightly differently. It describes a miracle that took place in 1263 (which we needn't necessarily be concerned with, but you can follow my link if you're interested) and adds that Thomas Aquinas was commissioned by Pope Urban IV "to compose the Proper for a Mass and an Office honoring the Holy Eucharist as the Body of Christ."
Corpus Christi Catholic Church in Phoenix, Arizona (which is where I was introduced to Carlo Acutis and his website) asks why Corpus Christi should be celebrated on a Thursday, and suggests "Perhaps as a reminder that Jesus celebrated the Last Supper on the Thursday before His passion."
This doesn't explain why it's the Thursday after Trinity Sunday, which (as its name implies) celebrates the Christian doctrine of the three Persons of God: the Father, the Son, and the Holy Spirit. Acutis states that Pope Urban IV "introduced the saint's composition" in August 1264, but this would seem to be when he added the feast to the calendar, rather than when it was first celebrated; Pentecost, Trinity Sunday, and the following Thursday, are unlikely to be in August.
Carlo Acutis was born in London, into an Italian family, in 1991. The family returned to Italy four months later. Carlo died from leukaemia in 2006, aged 15. Described as a "computer geek", he created the website referred to above to catalogue Eucharistic miracles and Marian apparitions.
© Haydn Thompson 2024