Here is an excellent sermon by a traditional priest on the origins and custom of Rogation Days:
The Rogation Days are treated on this site in the article “Sanctifying the Earth, and Our Parishes,” by Charles A. Coulombe. There is also this informative piece on the Fisheaters web site. Lastly, there is a short write-up on this site for the feast of Saint Mamertus (477), who began the custom in the late fifth century; his feast-day is May 11, which can occur on the Rogation Days, as it did this year. (These days are attached to the Paschal cycle, so they are movable liturgical days, not fixed to specific dates.)
