In it, he does make it clear how to work out when Septuagesima falls, but it passes by rather quickly and in the midst of other information, so I thought it might be helpful to spell it out.
You start by finding Ash Wednesday, which is normally easy to establish from any Catholic diary, yearbook, or even Google. Then you work backwards from that: the Sunday before Ash Wednesday is Quinquagesima, the one before that Sexagesima, and the one before that Septuagesima.
From which you will readily see that tomorrow is Sexagesima.
So here is the hymn Attende Domine:
Dextera Patris, lapis angularis, via salutis, ianua caelestis, ablue nostri maculas delicti.
Rogamus, Deus, tuam maiestatem: auribus sacris gemitus exaudi: crimina nostra placidus indulge.
Tibi fatemur crimina admissa: contrito corde pandimus occulta: tua Redemptor, pietas ignoscat.
Innocens captus, nec repugnans ductus, testibus falsis pro impiis damnatus: quos redemisti, tu conserva, Christe.
Hearken, O Lord, and have mercy, for we have sinned against Thee.
Crying, we raise our eyes to Thee, Sovereign King, Redeemer of all. Listen, Christ, to the pleas of the supplicant sinners.
Thou art at the Right Hand of God the Father, the Keystone, the Way of salvation and
Gate of Heaven, cleanse the stains of our sins.
O God, we beseech Thy majesty to hear our groans; to forgive our sins.
We confess to Thee our consented sins; we declare our hidden sins with contrite heart; in Thy mercy, O Redeemer, forgive them.
Thou wert captured, being innocent; brought about without resistance, condemned by impious men with false witnesses. O Christ keep safe those whom Thou hast redeemed.