By the Middle Ages, the first thirty days after the departing of a soul were known as the "minding days," at which time one remembered the deceased; that is to say, "had them in mind." One could have a Requiem Mass said on the third and seventh days after, or even have a series of "Gregorian Masses" said, one Requiem Mass each day for the thirty days, usually by an order or institute dedicated to such work. The thirtieth day after one's passing marked the end of the "minding days," and so its Mass was known as a “Month’s Mind.” During penitential seasons such as Lent, these Masses, or one to mark the anniversary, were the only ones permitted other than the Mass designated in the Ordo for that day.
So tonight, in the chapel of a local rectory, thirty days after entering into Eternity, Dad will get the Mass he would have wanted for much of his life, as a private Low Requiem Mass (the traditional form, in Latin) will be said in his memory. “Missa Defunctorum In Die Trigesimo” is its official name. In attendance will be only a priest, assisted by an acolyte in the form of yours truly, both joined by the choirs of Angels and Saints.