In the Roman Catholic liturgical calendar, there are two days that specifically honor the Holy Eucharist: Holy Thursday and Corpus Christi (The Body and Blood of the Lord).
These two feasts are separate and while they do share some similarities, they have many differences as well.
Holy Thursday
Pope Benedict XVI contrasted the two feasts in his homily on Corpus Christi in 2011:
The Feast of Corpus Christi is inseparable from Holy Thursday, from the Mass in Caena Domini, in which the Institution of the Eucharist is solemnly celebrated. Whereas on the evening of Holy Thursday we relive the mystery of Christ who offers himself to us in the bread broken and the wine poured out.
Often the Mass on Holy Thursday is called, "The Mass of the Lord's Supper," and so immediately recalls to our minds the day when Jesus instituted the Eucharist.
Furthermore, it is a day mixed with sorrow, as it ends with a procession to a "garden," reminding everyone of Jesus' agony in the garden on the night before he died.
Corpus Christi
When it comes to the feast of Corpus Christi, Pope Benedict XVI framed it in the following way:
[O]n the day of Corpus Christi, this same mystery is proposed for the adoration and meditation of the People of God, and the Blessed Sacrament is carried in procession through the streets of the cities and villages, to show that the Risen Christ walks in our midst and guides us towards the Kingdom of Heaven.
Furthermore, St. Juliana's original vision gave a reason behind the new feast:
Finally Jesus told her, “The moon represents the ecclesiastical year. The dark spot within its shining surface means that there is still one feast missing. It is My will that a great festival be instituted in honor of My Sacred Body. Holy Thursday is more a day of sorrow than of joy. Now go and announce it to the world.” The humble nun shrank from such a task and told our Lord so, “Lord,” she said, “I am only a simple maiden and I am not at all worthy. So please entrust this work to holy and learned priests.”
In this context, Corpus Christi is meant to be a feast full of joy, connected to the resurrection of Jesus and how he gave to us the Eucharist as a gift.
