 St. Charles Borromeo speaks as if this practice of praying for forty hours was very ancient; and he refers it to the forty hours that Christ's Body remained in the tomb.The number 40 is also associated with the rain at the time of the flood, years on the way to the Promised Land, and Jesus fasting for 40 days before beginning his public ministry.The practice of reserving the Blessed Sacrament with some solemnity during the Easter Triduum began in the 12th or 13th century.