"Written in 1916, Leontovychs Shchedryk tells the story of a happy spring omen. A swallow flies into the house to herald a prosperous new year for the familyhealthy livestock, money, and a beautiful wife. In Ukrainian, the word Shchedryk is derived from shchedryj, meaning bountiful. Leontovych borrowed just four notes from the original folk melodythe motive that became the opening melody of the piece. He then expands on this motive, harmonizing and developing it over the course of the piece.
During this time, Ukraine was in the midst of great political and social unrest as it was dragged into World War I. Ukrainian soldiers were initially divided, some fighting for Austria-Hungary and the Central Powers, while most served under the Russian Imperial Army. When Imperial Russia collapsed as a result of the Russian Revolution in 1917, the focus of the conflict shifted to the Ukrainian War of Independence, culminating in the brief formation of an independent Ukrainian government.
By 1919, Koshyts, the conductor who commissioned Shchedryk, began taking the piece on tour around the world with the Ukrainian National Chorus, promoting Ukrainian music and culture. The group performed over 1,000 concerts across Europe, North America, and South America.
On October 5, 1921, the Ukrainian National Chorus performed Shchedryk before a sold-out audience at Carnegie Hall. Peter J. Wilhousky, an American composer, music educator, and choral conductor of Ukrainian descent, attended the Carnegie Hall concert and wrote the English version of the lyrics, passing it along to NBC Radio in 1922. The rest, as they say, is history. Wilhousky continued to produce choral arrangements throughout his career, creating translations and arrangements of music from the Slavonic liturgy and a popular arrangement of The Battle Hymn of the Republic, all of which are still in use today. Wilhousky is also remembered for establishing the All City High School Chorus in 1936, a chorus of some 250 talented high schoolers from all five boroughs of New York."
https://www.kdfc.com/articles/the-ukrainian-folk-tune-behind-carol-of-the-bells