https://www.newliturgicalmovement.org/2015/08/the-calculation-of-september-ember-days.html
New Liturgical Movement: The Calculation of the September Ember Days (Reprint)
Sacred liturgy and liturgical arts. Liturgical history and theology. The movements for the Usus Antiquior and Reform of the Reform.
new liturgical movementember dayscalculationseptemberreprint