As the priest was vesting for Mass this morning, he turned to me as if having had an epiphany of his own. While his scheduled homily was well prepared (and an audio clip thereof will be provided soon), he realized the prospect of a treatise on the Holy Family as an earthly Trinity.
I must confess to never having heard of the concept. So I looked it up, and learned of the work of Jean Gerson (1363-1429), chancellor of the University of Paris.
Devotion to the Holy Family first appears in the late Middle Ages ... there was no word or expression to denote nuclear family. At the time, the word "family" was understood to mean "extended family" (or all those living under the authority of the paterfamilias), rather than "nuclear family," as it does today ... [Gerson] made it possible to speak about the Holy Family by coining the expression "earthly trinity" to speak about Jesus, Mary, and Joseph as a nuclear family in the modern sense ... it became popular to refer to and to depict in the visual arts the Holy Family as a trinity which replicated on earth the blessed Trinity in heaven. (Fr Joseph F Chorpenning, OSFS, "The Holy Family Devotion: A Brief History")