In the 5th century, St. Mamertus the Bishop of Venice appointed three days of public prayers and processions for his diocese in consequence of certain great calamities such as fire, earthquakes and other scourges which were devastating his country.  During these processions, Litanies were recited to appease the anger of God.  This custom later became universal, and the three days before the Feast of Our Lord's Ascension were set aside by the Church as a time for these public prayers.  Even before St Mamertus, it was customary in Rome to have processions of this kind on St. Mark's day, April 25.  The Rogation Days are special days of intercession especially for the seasonal planting of crops.  

The most appropriate and commonly used Litany for this time is the Litany of the Saints.

Link here: https://op54rosary.ning.com/profiles/blogs/litany-of-the-saints

Certainly at this time in our sad world, this time of prayer and penance is more needed than ever - that God may be merciful to us poor sinners.

Source:  The prayer book "Blessed Be God" by Very Rev. Charles Callan, O.P and Very Rev. John McHugh, O.P.