Is there guidance in our liturgical books for when to use the “regular” versus “solemn” tones? Obviously for big feasts the “solemn” would be used, but what about, say, “green” Sundays?
As I recall, in the EF, "solemn" tones are used on all Sundays and holydays. "Ferial" tones are for ferias (doh) and Requiem Masses. You can find instruction on P. 98 of the Liber Usualis
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