From the missal:
“Earthquakes and other calamities afflicted the diocese of Vienne in Dauphiny (France) in the fifth century, and St. Mamertus, who was bishop of that diocese, instituted a penitential procession with public supplications on the Monday, Tuesday, and Wednesday before Ascension Day. In 816, Pope Leo III introduced it into Rome, and soon after it became a general observance throughout the Church.
“The Litany of the Saints, the Psalms, and the Prayers sung during the procession on these days are supplications: hence the name of Rogation Days (rogare, to ask) applied to them. The object of these rogation supplications is to appease the anger of God and avert the scourges of His justice, and to pray for the harvest.
“A similar function is observed on April 25, on the feast of St. Mark, but this is of Roman origin. It is called the Greater Litanies in contrast to the Lesser Litanies of the Rogation days; but in practice there is no difference between them, except that the Rogations may be transferred by the Bishop of the diocese to three other continuous days which are more convenient according to local custom or need.”
The Online Breviary has the Latin/English text of the Litany.