The event — on January 19 on the Orthodox calendar — is frequently marked with a dip in icy waters to symbolize the baptism of Christ in the River Jordan.
But in addition to washing away their sins, many worshipers also welcome the midwinter submersion as a test of hardiness and good health.
The Russian Orthodox Church, with around 90 million members, traditionally carries out the blessing of the waters twice, on the eve of Epiphany and again on the holiday.
Russian Orthodox Patriarch Kirill celebrated Epiphany Eve with a ceremony at the Church of Christ the Savior Cathedral in Moscow, the church said.
Thousands of Musovites had already plunged into outdoor rivers, ponds, or other pools well before midnight on January 18, according to emergency authorities quoted by TASS, with temperatures remaining unseasonably warm at just a few degrees below zero and thousands more expected to turn out overnight and on January 19.