The Rogation Days are the Monday, Tuesday, and Wednesday before Holy Thursday, or the Ascension of our LORD.
About the middle of the fifth century, Mamericus, Bishop of Vienne, upon the prospect of some dreadful calamities that threatened his diocese, appointed that extraordinary supplications, accompanied with fasting, should be offered up to GOD, on the Monday, Tuesday, and Wednesday before the festival of the Ascension. Rogations being the Latin name given to these supplications, the days on which they were offered were called Rogation days; and the observance of them soon became general in the Church.
The design of the Church, in enjoining these days to be observed, is, not only to prepare us to celebrate with proper devotion our Saviour’s ascension; but also to appease GOD’S wrath, that so he may be pleased to avert the judgments which our sins have deserved; and that he may vouchsafe to bless the fruits with which the earth is at this time covered.
[Excerpt from John Henry Hobart, A Companion for the Book of Common Prayer, Containing an Explanation of the Service, 100-101. 1859.]