Quoted from Catholic Answers [1]:
Corpus Christi (Body of CHRIST), FEAST OF, is celebrated in the Latin Church on the Thursday after Trinity Sunday to solemnly commemorate the institution of the Holy Eucharist.
Quoted from Wikipedia [2]:
The Feast of Corpus Christi (Ecclesiastical Latin: Dies Sanctissimi Corporis et Sanguinis Domini Iesu Christi, lit. 'Day of the Most Holy Body and Blood of Jesus Christ the Lord'), also known as the Solemnity of the Most Holy Body and Blood of Christ, is a Roman Catholic, Anglican, and Western Orthodox liturgical solemnity celebrating the Real Presence of the Body and Blood, Soul and Divinity of Jesus Christ in the elements of the Eucharist.
Quoted from The Catholic Thing "(Excerpted from his [Pope Benedict XVI's] General Audience of November 14, 2010)" [3]:
This morning too I would like to introduce a female figure [Saint Juliana of Liège] to you. She is little known but the Church is deeply indebted to her, not only because of the holiness of her life but also because, with her great fervor, she contributed to the institution of one of the most important solemn Liturgies of the year: Corpus Christi.
[1] https://www.catholic.com/encyclopedia/Feast-of-Corpus-Christi, without hyperlinks.
[2] https://en.wikipedia.org/wiki/Feast_of_Corpus_Christi, without reference and hyperlinks.