"This is our reason for drawing the attention of our readers to certain beauties of the Divine Office, which would otherwise be totally ignored. Thus, what can be more impressive than this solemn Invitatory of to-day’s Matins, which the Church takes from one of the psalms, and which she repeats on every feria between this and Maundy Thursday? She says; To-day, if ye will hear the voice of the Lord, harden not your hearts! The sweet voice of your suffering Jesus now speaks to you, poor sinners! be not your own enemies by indifference and hardness of heart. The Son of God is about to give you the last and greatest proof of the love that brought Him down from heaven; His death is nigh at hand: men are preparing the wood for the immolation of the new Isaac: enter into yourselves, and let not your hearts, after being touched with grace, return to their former obduracy; for nothing could be more dangerous. The great anniversaries we are to celebrate have a renovating power for those souls that faithfully correspond with the grace which is offered them; but they increase insensibility in those who let them pass without working their conversion. To-day, therefore, if you hear the voice of the Lord, harden not your hearts!"
-The Liturgical Year by Dom Prosper Gueranger