In the Catholic Liturgical calendar, today is celebrated as the Feast of the Assumption of the Blessed Virgin Mary into Heaven. In the Middle Ages, when the veneration of Mary as Notre Dame – French for "Our Lady" – was at its peak, a "Lady Mass" would be sung on a day like this.

And it's quite likely that one of the earliest-known settings of the Latin mass, the "Notre Dame Mass" by Guillaume de Machaut, was performed as a Lady Mass at one particular chapel in the Cathedral of Reims for many years in the 14th century. Guillaume and his brother Jean were both canons at that Cathedral and had arranged an endowment for a mass in honor of Mary to be sung there every Saturday.

In our day, Guillaume de Machaut's Notre Dame Mass is his most famous work, but in his own time, the age of Chaucer, Petrarch, and Boccaccio, he was far better known as a secular poet of Courtly Love. Machaut had been a widely-travelled and extremely well-connected artist before returning to his native Reims at the end of his life. Before that, employment by various members of the royalty took him from Paris to Prague and on trips to Italy, Poland, and Lithuania.

It's ironic that Machaut is nowadays famous for his sacred music – this one Mass in particular – when the vast majority of his music was decidedly secular in tone.