The Passover meal has been completed in one sense. And yet in another sense, the Passover is far from finished. What was this hymn? It may have been several Psalms, perhaps including Psalms 113-118. Consider praying these Psalms as part of your daily prayer.
The Mt. of Olives is rich with theological depth (see Zechariah 14:4 as well as 2 Samuel 15:30 [King David weeps, as will the Son of David] and by allusion in 1 Kings 11:7, 2 Kings 23:13, Nehemiah 8:15 and Ezekiel 11:23).
This text contains both foreboding and promise. The flock will indeed be scattered (Zechariah 13:7). But, Christ Jesus promises, I will go ahead of you to Galilee.