The YouTube description from last year here notes:
Muscovites braved minus 19 degree Celsius weather on Sunday (January 18) and plunged themselves into icy water to celebrate the Orthodox Epiphany.
About a hundred people gathered on the frozen Moskva river in the west of Moscow for a prayer and a ritual dive. After a priest blessed the water, people formed a queue to the hole cut in the ice and started dipping in to the freezing water.
“Now all water is blessed and you get blessed yourself, all water in the whole world is blessed today,” said Alexander from Moscow, after dipping three times into icy water.
“It gives you strength, first of all spiritual and then body strength, it goes from spirit to into the body, if spirit is weak, we are weak, as the saying goes “it is not that our enemies are strong, but it is us who are weak.” We need to get stronger,” said Tatyana. “It’s quite cool. But spirit, spirit and belief, that’s what it is about, it gives you luck in life,” said another Alexander as he ran to a warm changing cabin.
The ritual commemorates the baptism of Jesus Christ in the Jordan River, or the Epiphany, which the Russian Orthodox Church mark on 19 January, in the middle of the Russian winter.
Traditionally the celebration starts the night before. By bathing on this day, believers symbolically wash off their sins. Many, however, also believe the dip in the cold water is good for their health.
You can read more here. Meantime, watch Muscovites take the plunge below.