They were married 30 April 771, the same year that he repudiated Desiderata and sent her home, and months before his brother and co-ruler Carloman died. She came from Carloman's territory, and so Charlemagne may have married her for the political value of having an ally in the midst of his brother's (and rival's) land.
Because the Franks and the Church followed Roman law, a girl reached marriage age at 12; it is thought that Hildegard was likely 12 or 13 when they were wed. She went through eight pregnancies (they had twins) between 771 and 783. Charlemagne took her on campaign with him; their first daughter, Adelaide, was born at the Siege of Pavia. She also went with him and the family to Rome in 780-81, where their sons Louis (later "the Pious" and king) and Carloman (named after Charlemagne's brother but renamed "Pepin" at Rome) were baptized by Pope Adrian I. A 2019 biography of Charlemagne expresses the belief that Hildegarde was interested in astronomy and is referred to as the "Astronomer" in a chronicle about Charlemagne's campaign in Spain.
Hildegarde was devout: she was a friend of St. Leoba, and made many donations to the monasteries of St. Denis, St. Martin of Tours, and others. A Hildegarde Chapel at the Monastery of Kempten commemorates her, and they mention her as a founder. She managed to gain benefits for her siblings through her husband. She is the only wife of Charlemagne who was still married at the time of her death. She died on 30 April 783 and was buried on May Day in the Abbey of Saint-Arnould in Metz.
This series started yesterday on International Women's Day, but is far from a one-day sequence. More tomorrow.