The Chrism Mass is the special Liturgy where the bishop of a diocese will bless the Oils (also called chrisms) that will be used in the various Sacraments throughout the year. In many places it is celebrated on Holy Thursday morning (that is when the Pope celebrates it) and it gathers all the priests of the diocese together with the bishop. Holy Thursday is the day when we commemorate both, the institution of the Mass, and of the Priesthood.