Mythic story of walking through a year is an ancient Slavic myth which primarily carries metaphorical knowledge about season changes in nature, which mainly applied to agriculture (because our ancestors depended on it).
The myth itself has 2 levels: one is a cosmic clash/conflict between Perun, god of sky and thunder, and Veles, god of underground. On the other level, myth is a cyclic repeating story which follows the main actor Jarilo and his walk through one year.
The myth is cyclic because, as a difference from Christian linear concept of time, our ancestors understand time in a repeating manner, new cycle started with birth of Sun.
On the day of winter solstice Perun and his wife Mokoš had a son Jarilo (young Sun). In parallel to the myth, in our “earthly” understandings, summer solstice marks the end of long, dark days, and the beginning of work in fields. Jarilo grows up in Perun’s heavenly palace. Not long after his birth, Koledars (representations of deceased ancestors; from Veles’s world) are coming to pay him a visit and to bow to the new god. The custom was to give gifts to Koledars, but Perun refused to do so, so they kidnapped his son, young Jarilo, and took him to underground, to Veles.
Jarilo stayed with Veles and basically grew up in underground, not knowing that his real dad was Perun. One day (on the first day of spring) he decides to leave the underground. He does that by defeating Veles (some versions say that he beats him philosophically by looking him keys to the underground, and the other one is Jarilo killing him). Veles is the symbol of not only ancestors and underground, but also water, so with Jarilo defeating him, he released water to the world, and that means vegetation and fertility to our world. In our tradition, some of the customs in celebrating Jarilo are spring bonfires.
On his journey through ‘our world’, Jarilo meets Morana, a young beautiful girl (who is also his sister, but they don’t know that since Jarilo grew up in the underground). They soon fall in love, and marry. Metaphorically, their wedding means joining two worlds, sky and underground, fire and water. Now, don’t get cringey on their sibling wedding, bare in mind that in world of gods is perfectly normal for them to mix, because after all they are gods and they are allowed to do thing which is not allowed to humans! Their wedding also marks the beginning of summer and the peak of natures blossom and fertility.
Not long after their wedding, Jarilo cheats on Morana, so she, driven by anger and grief, murders him with fathers attributes, arrows! Sadness completely overwhelms her, so she becomes an old lady which brings winter. That means the end of working in the fields. She dies in February, and her death marked beginning of ancient slavic new year and opening space to a new vegetation cycle!