According to the Catholic Church's Canon Law, we are obliged to attend Mass on certain days every year:
Can. 1246 §1. Sunday, on which by apostolic tradition the paschal mystery is celebrated, must be observed in the universal Church as the primordial holy day of obligation. The following days must also be observed: the Nativity of our Lord Jesus Christ, the Epiphany, the Ascension, the Body and Blood of Christ, Holy Mary the Mother of God, her Immaculate Conception, her Assumption, Saint Joseph, Saint Peter and Saint Paul the Apostles, and All Saints.
Here in the United States, however, the solemnities of Saint Joseph and Saint Peter and Saint Paul the Apostles are not Holy days of Obligation, because:
§2. With the prior approval of the Apostolic See, however, the conference of bishops can suppress some of the holy days of obligation or transfer them to a Sunday.
So many Catholics outside of the U.S.A.--including in the dioceses of England and Wales--are celebrating this feast tomorrow as a Holy day of Obligation, also curtailing their workaday activities as though it is a Sunday. Pope Francis will celebrate Mass and bless the Pallia for the new Cardinals in Vatican City at St. Peter's Basilica.
Tonight, however, I'll have the opportunity to attend Mass for the Vigil of the feast in the Extraordinary Form of the Latin Rite (EFLR) and tomorrow in the Ordinary Form of the Latin Rite at daily Mass. It's interesting to note that the vestment color for tonight's Mass in the Extraordinary Form is violet, while the vestments for the Ordinary Form is Red tonight for the Vigil and also tomorrow--Red is the color of the EFLR Mass tomorrow. Even though the Solemnity is not a Holy day of Obligation in the U.S.A., it's clear that it's a special feast. If one went to Mass during the day today, it's the Memorial of St. Irenaeus of Lyon, Bishop and Martyr. Tonight, we begin celebrating the feast of Saint Peter and Saint Paul with Vespers and the Vigil Mass.