Aleksandr Ostrovskiy wrote the outline for operetta "Ivan-Tsarevich" ("Prince Ivan") in 1860s - 1-2 paragraphs per actor per scene - and drafted first 7 parts out of 16. It wasn't finished, because the theater decided to commission a musical fairytale from a different playwright. Yuliy Kim reworked the draft for staging in 1979, removing a number of characters and scenes and adding his own songs and the final.
The play was staged as "Ivan-Tsarevich" ("Prince Ivan") in Mayakovsky Theatre in Moscow in the early 1980s. The plot remains the same, but there are a number of differences from the movie. For example, in the play Solovey-Razboynik and Babadur are introduced fairly early, Solovey is not mute, brothers work in a small grain mill instead of a large quarry.
The title means "never", something like "when hell freezes over". The origin of the phrase is obscure. One possible explanation is that prayers to Perun (god of thunder, similar to Jupiter) to bring rain during drought are most likely to come true on Thursday. But they never come true.