The last sentence suggests a certain order in the promotion of the clergy: they should be promoted according to the period spent in lower office.
The times set for ordinations are what later came to be known as Ember Days, the time set for fasting during each of the four seasons of the year. "The middle of Lent" may be the "Dominica Mediana", the fifth Sunday of Lent, which was the station at St Peter's. The Ember Saturdays are confirmed as the traditional days for the ordinations in other sources: The Gelasian Sacramentary and Ordo Romanus IV (see Jeffrey 2013: 175).
The same information is given in paragraph 3 of Letter 15 of Gelasius (ed. Thiel 1868: 380), which is the general formulary for his letters, and applied in his Letter 16 (ed. Thiel 1868: 381).