This is a follow-on to Carol Marie’s question. Is there a difference between a Holy Day and a Day of Solemnity?
“Holy Day” is short for “holy day of obligation” which is treated as a Sunday and the faithful are bound under pain of mortal sin to attend Mass.
A Solemnity is the highest rank given to a celebration of our Lord or one of the saints. All holy days of obligation are solemnities. Most solemnities are not holy days of obligation, e.g., the Solemnity of St. Joseph.
Fr. Vincent Serpa, O.P.