The Chrism Mass is where the oils used by priests are blessed.
Usually it is on the morning of Holy Thursday. From the Ceremonial of Bishops:
“275. The blessing of the oil of the sick and the oil of catechumens and the consecration of the chrism are ordinarily done by the bishop at the Chrism Mass celebrated on Holy Thursday morning.
If it is difficult for the clergy and the people to assemble with the bishop on Holy Thursday morning, the blessing may be held on an earlier day, near Easter, with the celebration of the proper Chrism Mass.
276. Because of its meaning and pastoral importance in the life of the diocese, the Chrism Mass should be celebrated as a stational Mass in the cathedral church, or for pastoral reasons, in another church.”
Earlier in the Ceremonial of Bishop it has:
“119. … Hence, as many of the faithful as possible should come together for a stational Mass, priests should concelebrate with the bishop, deacons should assist in the celebration, and acolytes and readers should carry out their ministries.”
So people are encouraged to attend.