These are days of penance and supplication, traditionally observed with solemn processions during which the Litany of the Saints is recited. There are four Rogation Days in the traditional calendar of the Latin Church: the Major Litany observed on the Feast of Saint Mark (25 April), and the three Minor Litanies observed on the three days preceding the Feast of the Ascension.

From The Liturgical Year by Dom Prosper Gueranger, OSB :

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 must, by humbling ourselves and invoking the intercession of the Mother of God and the Saints, obtain the health of our bodies and preservation of the fruits of the earth; we have to offer atonement to 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 their number.

A day, then, like this, of reparation to God’s offended majesty, would naturally suggest the necessity of joining some exterior penance to the interior dispositions of contrition which filled the hearts of Christians. Abstinence from flesh-meat was long observed on this day at Rome … a council held at Aachen in 836 enjoined the additional obligation of resting from servile work on this day.