Holy Thursday is the first day of the Triduum. This day celebrates the institution of the Eucharist as the true body and blood of Jesus Christ and the institution of the sacrament of the priesthood.
At the conclusion of the Mass, the faithful are invited to continue Adoration of the Blessed Sacrament throughout the night, just as the disciples were invited to stay up with the Lord during His agony in the garden before His betrayal by Judas.