This article explores a practice in evidence across Europe from the twelfth to the nineteenth century involving the singing of a brief refrain within sacred Latin songs and hymns. Tracing the circulation of the two-part refrain “Fulget dies … Fulget dies ista” across multiple centuries, in both song-form tropes of the office versicle Benedicamus Domino and as a trope interpolated into hymns, I chart its unique movement between genres and in and out of written record. Examining the unusual origins, transmission, and function of the refrain, I begin with its emergence in twelfth-century manuscripts and conclude with its unnotated appearance in nineteenth-century printed Catholic songbooks. I argue that the refrain’s long-standing appeal can be located in its function as a poetic and liturgical trope of time itself. While tropes often enhance the “hic et nunc” (here and now) of the liturgy, the “Fulget dies” refrain gained additional temporal significance through its intimate link to songs of the Christmas season. The “shining day” imagery introduced by the refrain offered a tangible way of marking seasonal time in devotional rites, poetically indexing the light-based symbolism of Christmas, the winter solstice, and the New Year. The inherently temporal meaning of the refrain lent it flexibility as a trope, enabling its movement across genres and liturgies. Integrated into sacred Latin songs, the “Fulget dies” refrain functioned as a pithy musical and poetic commentary on liturgical, calendrical, and seasonal temporalities—in other words, as a trope of time in sacred song.