Epiphany is among the most ancient of Christian holidays, marking the end of the twelve days of Christmastide and, for western Christians, celebrating the Magi who discovered the infant Jesus by following the eastern star. Later in life, Karl Straube would refer to this day as “C+M+B Tag,” recalling Caspar, Melchior, and Balthasar, the traditional names of the Eastern sages who appeared at Christ's manger. In the German lands, a number of folk traditions had grown up around the festival, including door-to-door singing and a three-kings-cake. These activities would have been no less in evidence in 1873, when Epiphany fell on a Monday. But on that day, January 6, the Berlin home of Johannes and Sarah Palmer Straube was consumed with a matter of greater urgency, the birth of the couple's second and last child, a boy given the name Carl Mondgomery Rufus Siegfried and baptized on February 18. His older brother, William Carl Johannes Bertram, had arrived on June 10, 1871.

The Straube family resided at Wilhelmstraße 29 in central Berlin, not far from the Tiergarten and what later became the famous Checkpoint Charlie, on the prestigious street that hosted the offices of the imperial government, foreign embassies, and other administrative entities. The house at number 29 served as the parsonage for the nearby Bohemian-Lutheran Bethlehemskirche, no longer extant. The paternal heritage emphasized music and theology: Karl and William's great-great-grandfather Johann Augustin Straube (1725–1802) had been a noted instrument builder esteemed by Forkel, E. L. Gerber, and Kirnberger, originating from Alt-Brandenburg and active in Berlin by the mid-eighteenth century. By 1786 he lived in the Mohrenstraße, near the Wilhelmstraße. J. A. Straube's son Carl Augustin Friedrich was the Lutheran superintendent-provost in Mittenwalde, a town in the Spreewald south of Berlin. His marriage to Dorothea Knak in 1806 produced a son, Carl Augustin Friedrich Victor, Karl's grandfather (1807–1881). Ordained in 1835, he became pastor first in Werder, near Potsdam, then in 1856 in Falkenhagen, east of Berlin near Frankfurt (Oder). In Werder he founded a pietistic “Bible Reading Association” and a “Bible Society,” for which he edited an annual comprehensive index of biblical passages for the liturgical year. Wolgast undoubtedly repeats Karl's own recollections in referring to the paternal grandfather as “likewise a theologian and orthodox Lutheran by conviction, admittedly with a strongly romantic-pietistic element.”