Russians are the only people in the world who do not celebrate their fortieth birthday. Why?
Negative number
Some commentators explain this superstition by the fact that the number 40 has a lot of negative associations. The Flood lasted 40 days. For 40 days, according to the Orthodox tradition, the soul wanders around the Earth before going to heaven or hell. For 40 years the prophet Moses led the Jewish people in the desert until he brought them to the promised land. The last episode, by the way, is quite positive.
Abundant version
The word "forty" got into Russian from the Polish language, and meant specifically a bunch of fur skins. Then, in the Russian language element, this word meant a kind of abstract set ("centipede", "forty forties") and abundance. Superstition probably arose as a protective form, so as not to attract evil or evil eye in the most "abundant" year of life.
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Martyr's version
In Orthodox Russia, the Forty Martyrs of Sebastia, Christian warriors who were martyred for their faith, were especially honored. The day of their memory was one of the most revered Orthodox holidays and even received the informal name "Magpies" among the people. Probably because of this, the number 40 acquired a certain “martyr’s” subtext in the Russian tradition.