Sirach 35:10
The Divine Liturgy is said most of the Liturgical Year.
While normally said Penitential times of the Year,
this Divine Liturgy is not one of penitence.
This is also referred as the Liturgy of the Presanctified Gifts.
It is celebrated on Days of Fasting, never on Saturdays & Sundays.
Below are worthwhile links to go more in-depth.
Each link has references to additional material, online or in print.