Following from the last post let us dissect what the Ordo entry means for today.
The date and liturgical colour are self-explanatory. The double cross at the beginning of the entry indicates that today is a 'Day of Devotion' i.e. formerly a holyday that is no longer observed as such. As we noted above the feast is a double of the first class and has an Octave.
'Missa pro Populo' means that (according to the 1917 CIC) today is a day when all those to whom pastoral office is entrusted celebrate Mass for the flock committed to their care.
'Off pr' translates as the Office is proper. Looking at the liturgical books containing the Office this becomes clear. The feast has proper texts and antiphons. Mattins has three nocturns of nine lessons (which is usual for all doubles except Easter and Pentecost).
'Com Oct DNJC in L et M' tells us that the Octave of the Sacred Heart is commemorated at Lauds and at Mass. In the Office a commemoration is made by singing the antiphon, versicle and response and the collect of the commemoration. At Mass the collect of the Octave is sung after the collect of the feast.
'In M Gl, Cr (ratione Oct DNJC), Praef SS Cordis' means that in the Mass the Gloria is sung, the Creed is sung because of the commemorated Octave of the Sacred Heart (in the traditional rite St. John the Baptist does not have a Creed) and, as the Mass of St. John the Baptist does not have its own preface that of the occurring Octave is sung.
'V fest, com seq et Oct' translates as Vespers of the feast with a commemoration of the following Office of St. William and of the Octave of the Sacred Heart.
Is Ordo-speak that difficult?