- Major Rogation day: April 25th
- Minor Rogation days: the Monday, Tuesday, and Wednesday immediately before Ascension Thursday.
Rogation Days are days of prayer and 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."—Catholic Encyclopedia
rogation (n.) late 14c., from Latin rogationem (nominative rogatio) "an asking, prayer, entreaty," noun of action from past participle stem of rogare "to ask, question, propose, request," apparently a figurative use and meaning literally "to stretch out (the hand)," from PIE *rog-, variant of the root *reg- "move in a straight line"
Rogation days were the Monday, Tuesday and Wednesday before Ascension Thursday, a time for processions round fields blessing crops and praying for good harvest, also blessing the boundary markers of each parish.—Online Etymological Dictionary
An excellent visual guide to the ember and rogation days by Fr Christopher Smith: