Holy Thursday (specifically the evening mass) is the first day of the Easter Triduum, which includes the passion, death, burial, and resurrection of Jesus.
Holy Thursday is marked by events such as...
TAKE THIS, ALL OF YOU, AND EAT OF IT: FOR THIS IS MY BODY WHICH WILL BE GIVEN UP FOR YOU.
In a similar way, when supper was ended, He took the chalice, and giving You thanks He said the blessing, and gave the chalice to His disciples, saying:
TAKE THIS, ALL OF YOU, AND DRINK FROM IT: FOR THIS IS THE CHALICE OF MY BLOOD, THE BLOOD OF THE NEW AND ETERNAL COVENANT; WHICH WILL BE POURED OUT FOR YOU AND FOR MANY FOR THE FORGIVENESS OF SINS. DO THIS IN MEMORY OF ME.