Tomorrow, the feast of St. Mark, is also a day set aside for a penitential procession and was designated so long before St. Mark’s festival was instituted. As Dom Gueranger says in his history of the Greater Litanies,

Laden as we are with the manifold graces of this holy season, and elated with our Paschal joys, we must sober our gladness by reflecting on the motives which led the Church to cast this hour of shadow over our Easter sunshine. After all, we are sinners with much to regret and much to fear; we have to avert those scourges which are due to the crimes of mankind; we have, by humbling ourselves and invoking the intercession of the Mother of God and the Saints, to obtain the health of our bodies and the preservation of the fruits of the earth; we have to offer atonement of divine justice for our own and the world’s pride, sinful indulgences, and insubordination. Let us enter into ourselves, and humbly confess that our own share in exciting God’s indignation is great; and our poor prayers, united with those of our holy Mother the Church, will obtain mercy for the guilty, and for ourselves who are of the number.

The great feature of the day is the procession with the singing of the Litany of the Saints, followed by several versicles and prayers. You may want to recite them at home if you do not have a procession available to you. Find them here.

Here is a link to Dom Gueranger’s history and explanation of this day. You will find it by scrolling down to the heading, “St. Mark’s Procession”. The link also provides a bounty of information on St. Mark.