A three o'clock prayer is a Catholic devotion to Jesus, to remember his sacrifice and passion and beg his forgiveness. The prayer is made daily at three in the afternoon because that is the time on Good Friday that Jesus died on the cross.
The tradition of a three o'clock daily call to prayer was inspired by a Polish nun, Sister Faustina Kowalska, who received divine revelations in the 1930s. In the revelations, Sister Faustina was directed to record her experiences in a series of diaries now known as Divine Mercy in My Soul. Jesus told her specifically for all sinners to implore his mercy and immerse themselves in his passion at three o'clock each day. Sister Faustina died in 1938, was beatified in 1993 and canonized in 2000 by Pope John Paul II.