For an Abundant Harvest: Rogation Days, like their distant cousins the Ember Days, are days set aside to observe a change in the seasons. Rogation Days are tied to the spring planting. There are four Rogation Days: the Major Rogation, which falls on April 25, and three Minor Rogations, which are held on the Monday, Tuesday, and Wednesday immediately before Ascension Thursday.
As the Catholic Encyclopedia notes, Rogation Days are "Days of prayer, and formerly also of fasting, instituted by the Church to appease God's anger at man's transgressions, to ask protection in calamities, and to obtain a good and bountiful harvest."
Origin of the Word: Rogation is simply an English form of the Latin rogatio, which comes from the verb rogare, which means "to ask." The primary purpose of the Rogation Days is to ask God to bless the fields and the parish (the geographic area) that they fall in. The Major Rogation likely replaced the Roman feast of Robigalia, on which (the Catholic Encyclopedia notes) "the heathens held processions and supplications to their gods." Since the practice itself was laudatory, just misdirected, Christians adopted it as their own. By the time of Pope St. Gregory the Great (540-604), it was already considered an ancient custom.
The Litany, Procession, and Mass: The Rogation Days were marked by the recitation of the Litany of the Saints, which would normally begin in or at a church. After Saint Mary was invoked, the congregation would proceed to walk the boundaries of the parish, while reciting the rest of the litany (and repeating it as necessary or supplementing it with some of the penitential or gradual Psalms). Thus, the entire parish would be blessed, and the boundaries of the parish would be marked. The procession would end with a Rogation Mass, in which all in the parish were expected to take part.
Optional Today: Like the Ember Days, Rogation Days were removed from the liturgical calendar when it was revised in 1969, coinciding with the introduction of the Mass of Paul VI. Parishes can still celebrate them, and, in portions of Europe, the Major Rogation is still celebrated with a procession. As the Western world has become more industrialized, Rogation Days and Ember Days, focused as they are on agriculture and the changes of the seasons, have seemed less "relevant." Still, they are good ways to keep us in touch with nature and to remind us that the Church's liturgical calendar is tied to the changing seasons.
Celebrating the Rogation Days: If your parish does not celebrate the Rogation Days, there's nothing to stop you from celebrating them yourself. You can mark the days by reciting the Litany of the Saints. And, while many modern parishes, especially in the United States, have boundaries that are too extensive to walk, you could learn where those boundaries are and walk a portion of them, getting to know your surroundings, and maybe your neighbors, in the process. Finish it all off by attending daily Mass and praying for good weather and a fruitful harvest.