Bartók's study of Eastern European folk music was rigorous and comprehensive: by the 1930s, he had collected thousands of folk melodies, composed dozens of folk song settings, and incorporated folk tunes and styles into his compositions. Bartók, as of 1915, had already composed music using Hungarian and Slovakian folk tunes, and some scholars have suggested that this set of Romanian carols was composed for the sake of completeness: with the carols, the music of all three major ethnic groups in Hungary had been represented. These carols, or "colinde" are sung at Christmastime, but as Bartók himself noted, their texts do not necessarily relate to Christmas, but instead consist mostly of tales of ancient battles, myths, and folktales. Bartók also remarked on the tradition of singing these carols: groups of young men, after some rehearsal, would walk from house to house and, if received at a house, would sing a few carols in unison. Afterwards, they would be given a gift, and would then travel to the next house.
There are twenty Romanian Christmas Carols in this work, divided into two sets, each to be played without pause between songs (each song blends into the next). The work is comprised of original Romanian folk tunes, collected by Bartók over the course of many years, beginning in 1909. These pieces may have been composed by Bartók for teaching purposes: like his other collections of easy folk songs for beginning pianists, the carols combine relatively simple piano parts with what would have then been familiar folk tunes. While much of Bartók's folk-inspired music used peasant dance music as sources, the carols use only vocal music, and as such are very different in character from the dance-tune derived music. As a number of writers have noted, the most striking difference is in the flexible rhythms of the carol melodies, as opposed to the fixed rhythms of the peasant dances. This rhythmic flexibility, combined with asymmetrical melodic phrasing and the irregularity of the carols' text structure, meant that Bartók was compelled to compose equally flexible music. The carols, as a result, feature constantly shifting meters, what Bartók would later call "Bulgarian" rhythm; that is, the "unsystematic alteration" of meter. They are also less polyphonic than many of the composer's other works.
Bartók's harmonization of these tunes incorporates the carol melody into the accompanying chords. The original folk tunes, composed using Romanian scales and modes, lack a sense of closure: they are free-floating melodies that do not correspond to the cadential formulas of Western music. Bartók's settings do not diatonicize these tunes--that is, do not forcefully shape them into major or minor modes--but rather give the melodies more direction by setting them with a structured, balanced accompaniment.