We celebrate Rogation Days on Monday, Tuesday and Wednesday of this week. The following is adapted from The Catholic Encyclopedia at New Advent.
The Rogation Days are the 25th of April, called Major, and the three days before the feast of the Ascension, called Minor. The Major Rogation, which has no connexion with the feast of St. Mark (fixed for this date much later) seems to be of very early date and to have been introduced to counteract the ancient Robigalia, on which the heathens held processions and supplications to their gods. St. Gregory the Great (d. 604) regulated the already existing custom. The Minor Rogations were introduced by St. Mamertus, Bishop of Vienne, and were afterwards ordered by the Fifth Council of Orléans, which was held in 511, and then approved by Leo III (795-816). This is asserted by St. Gregory of Tours in "Hist. Franc.", II, 34, by St. Avitus of Vienne in his "Hom. de Rogat." (P.L., LVIII, 563), by Ado of Vienne (P.L., CXXIII, 102), and by the Roman Martyrology. Sassi, in "Archiepiscopi Mediolanenses", ascribes their introduction at an earlier date to St. Lazarus. This is also held by the Bollandist Henschen in "Acta SS.", II, Feb., 522. The liturgical celebration now consists in the procession and the Rogation Mass. For 25 April the Roman Missal gives the rubric: "If the feast of St. Mark is transferred, the procession is not transferred. In the rare case of 25 April being Easter Sunday [1886, 1943], the procession is held not on Sunday but on the Tuesday following".
The order to be observed in the procession of the Major and Minor Rogation is given in the Roman Ritual, title X, ch. iv. After the antiphon "Exurge Domine", the Litany of the Saints is chanted and each verse and response is said twice. After the verse "Sancta Maria" the procession begins to move. If necessary, the litany may be repeated, or some of the Penitential or Gradual Psalms added. For the Minor Rogations the "Ceremoniale Episcoporum", book II, ch. xxxii, notes: "Eadem serventur sed aliquid remissius". If the procession is held, the Rogation Mass is obligatory, and no notice is taken of whatever feast may occur, unless only one Mass is said, for then a commemoration is made of the feast. An exception is made in favour of the patron or titular of the church, of whom the Mass is said with a commemoration of the Rogation. The colour used in the procession and Mass is violet. The Roman Breviary gives the instruction: "All persons bound to recite the Office, and who are not present at the procession, are bound to recite the Litany, nor can it be anticipated".