E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The Monday, Tuesday, and Wednesday before Ascension Day. Rogation is the Latin equivalent of the Greek word Litany, and on the three Rogation days the Litany of the Saints is appointed to be sung by the clergy and people in public procession. (Litany, Greek litaneia, supplication. Rogation, Latin rogatio, same meaning.)