Some Slavic pagan reconstructionist groups have a special day for the Dark God on Leap Year Day. This day may be for Koschei, Chernobog, or the Dark Face of Veles.

Leap years are years that have an extra day, and they happen every 4 years, in our current calendar system. The extra day is February 29th, and some cultures have holidays for Leap Year's Day. One of those is the reconstructed pagan religion, Ukrainian Ridnoveri, and other Slavic pagan groups. But they use the Slavic calendar, which is the same as the Orthodox calendar in use in Slavic countries, based on the old Julian calendar, rather than the Gregorian calendar in use in Western countries. Slavic February 29th is not the on same day as our February 29th in the English speaking world. 

In 2024, February 29th falls on a Wednesday in the Julian calendar. In the Gregorian calendar, it's on a Thursday this year. The Julian calendar is 12 days behind the Gregorian calendar. To get the date of that day in the Gregorian calendar, add 12 days. Julian Leap Year Day is on Gregorian March 12. 

There are multiple names for the Slavic pagan holiday of Leap Years' Day, including: Кощей, Кощный Бог, Чернобог, Тёмный Лик Велеса. 

Ridnoveri groups and other Slavic pagan groups may or may not be marking this holiday this year. Those that do, may have very different customs depending on whether they call this the day of Koschei, the day of the Dark God, or the day of the Dark Face of Veles.

In fairy tales and modern fiction, Koschei is depicted as an antagonist character. However, some pagans consider him to be a modern version of Karachun, the lord of the dead. Koschei and Karachun are both associated with winter, cold, and death.

Veles is also a god of the underworld, but he had other spheres of influence as well. His Dark Face would also reference the realm of the dead. In the far north, winter is literally dark, without any sunshine. So, it is natural for winter and cold to be associated with darkness.

Veles is known to all Slavic cultures, sometimes called Volos. He is opposite to Perun and their sacred places were kept separate from each other, Perun's up high near the sky and Veles's below, closer to the underworld. The realm of the dead was seen as being a cold place in Slavic mythology and folklore.