They come at this time because of the Gospel reading for the previous Sunday (the 5th Sunday after Easter, sometimes for that reason called Rogation Sunday) which includes the text from the Gospel of St John 16:24 “Ask and ye shall receive”.
The Rogation days are:
(1) St Mark’s Day on 25 April (the Major Rogation)
(2) The 3 days before the Ascension Day (the Minor Rogations)
The first Rogation, which includes the Greater Litanies, was originally a christianisation of the old Roman pagan feast day of Robigalia, on 25 April, a day to pray for good crops.
The minor Rogations, including the Lesser Litanies, were introduced in 470 by Bishop Mamertus of Vienne and then spread to the rest of the Roman Church. They became a preparation also for the Feast of the Ascension.
These days coincided also with the ceremony of the “Beating of the Bounds” during which a procession led by ministers and choir would proceed round the boundaries of the parish, partly to re-affirm the boundaries but also to pray for the protection of the parish over the forthcoming year. Choir boys and servers would carry canes to "beat" the boundaries at each stopping point.
Absurdly, the Rogation days are yet further ancient and beautiful ceremonies that were done away with by the liturgical wreckers after 1970. They have, of course, come back with the return of the traditional rites.
The ceremony of the Beating of the Bounds can still be seen in the City of London and processions continue to go round the boundaries to the various churches of the City, often escorted by a marching group of one or more of the City TA Regiments.
What diabolical spirit came over the liturgical consilium of Archbishop Bugnini that he even thought to try and get rid of this beautiful, scriptural, ancient liturgical devotion? Yet further evidence, if it were still needed, that a dark and sinister spirit has, these 40 years past, been suffered by weak, vain and worldly prelates to invade parts of the Church.