The feast of St. Louis King of France is of semi-double rite.
As yesterday's Sunday liturgy was displaced by the feast of St. Bartholomew the Apostle at Mattins the 'Incipit' or beginning of the Book of Ecclesiasticus is read in the first nocturn of today's feast along with the preceding Sunday's responsories. The feast has nine lessons.
At Lauds the Suffrage of the Saints is sung and at Prime the Dominical preces are sung. At Mass the second collect is for the intercession of the Saints, A cunctis nos, and the third collect is of the celebrants choice. It is not a choice to omit the third collect but one may be chosen ad libitum. Many celebrants in the past chose Omnipotens, basing their choice on that mandated for certain days in Lent.
Vespers are of the feast with a commemoration of the following day's office, St. Zephyrinus. The Suffrage of the Saints is again sung and at Compline the Dominical preces are sung.