This was on the sacristy wall of the Convent of La Incarnación at Avila and attracted my attention. Similar inscriptions can be found throughout the sacristies of Catholic Europe. A rough translation would be: 'O Priest of Jesus Christ, celebrate this Holy Mass as if it were your First Mass, your Last Mass and your Only Mass.'
As the Holy Father writes in Sacramentum Caritatis (#94):
This most holy mystery thus needs to be firmly believed, devoutly celebrated and intensely lived in the Church. Jesus' gift of himself in the sacrament which is the memorial of his passion tells us that the success of our lives is found in our participation in the trinitarian life offered to us truly and definitively in him. The celebration and worship of the Eucharist enable us to draw near to God's love and to persevere in that love until we are united with the Lord whom we love. The offering of our lives, our fellowship with the whole community of believers and our solidarity with all men and women are essential aspects of that logiké latreía, spiritual worship, holy and pleasing to God (cf. Rom 12:1), which transforms every aspect of our human existence, to the glory of God. I therefore ask all pastors to spare no effort in promoting an authentically eucharistic Christian spirituality. Priests, deacons and all those who carry out a eucharistic ministry should always be able to find in this service, exercized with care and constant preparation, the strength and inspiration needed for their personal and communal path of sanctification...