I had an interesting liturgical question today about an old privilege for priests in the USA to be able to say a Requiem Mass on Mondays. I am pretty sure this is now defunct, but here is an article about it from the Catholic Encyclopedia of (1917)
The Monday Privilege
In the United States there is a faculty (“Fac. Ord.”, Form I, 20) ordinarily communicated to priests through the bishop which grants permission to celebrate a requiem Mass on Mondays non impeditis officio novem lectionem. The phrase officio novem lectionem gave rise to a doubt as to whether semi-doubles only were referred to, or if doubles also were understood. The Congregation of Rites answered (4 Sept., 1875, no. 3370, ad. 1) that this Mass was allowed on all Mondays during the year, except (a) on the vigils of Christmas and the Epiphany; (b) in Holy Week; (c) during the octaves of Christmas, the Epiphany, Easter, Pentecost, and Corpus Christi; (d) holy days of obligation; (e) greater doubles and doubles of the first and second class. If the enumerated cases hinder this Mass on Monday, the privilege is transferred to Tuesday, under the same conditions, but it lapses after that day.