On April 25, we celebrate the Feast of St Mark, but it is also a Rogation day. In Australia, we hardly ever get to celebrate fully this important and beautiful ceremonial, as ANZAC Day dawn services and requiems tend to overshadow or place the relevant mass and procession. But the useful fisheater's site gives the following description of what a Rogation day is, and you can read more about it over there:
""Rogation" comes from the Latin "rogare," which means "to ask," and "Rogation Days" are days during which we seek to ask God's mercy, appease His anger, avert His chastisements manifest through natural disasters, and ask for His blessings, particularly with regard to farming, gardening, and other agricultural pursuits. They are set aside to remind us how radically dependent we are on Mother Earth, and how prayer can help protect us from nature's often cruel ways..."
The rubrics in the 1963 breviary say that nothing is made of the Rogation Day in the Office, but only in the Mass, where, in accordance with the constitutions of the monastery or local custom, a procession is held at which the Litany of the Saints is sung (although the bishop can substitute other prayers).
Those who are bound to say the Office (such as priests and religious) but who can't participate in a procession say the litany and its particular prayers by themselves (or with other members of the faithful), generally immediately after Lauds. You can find a traditional version of the litany online here. You can also find them in the Monastic Diurnal at (200).