Cyril of Alexandria

But because the Pharisees thought that He who was the Lord of hosts, feared the hand of Herod. He refutes this, saying, Nevertheless I must walk to day and to morrow, and the day following. When He says must, He by no means implies a necessity imposed upon Him, but rather that He walked where He liked according to the inclination of His will, until He should come to the end of the dreadful cross, the time of which Christ shows to be now drawing near, when He says, To day and to morrow.

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

"Nevertheless, I must... today and tomorrow," that is, for some time, remain here and perform miracles, and "the following day" go to Jerusalem, for I have appointed for Myself to suffer there. Since they (the Pharisees) said to Him: "Get out..., for Herod wants to kill You," — and they said this in Galilee, which Herod governed, He declares to them that even with Herod's strong desire, he cannot kill Him, for He is appointed to suffer not in Galilee, but in Jerusalem. …

Commentary on Luke ↗

Bede

Nevertheless, I must walk today and tomorrow, and the next day, for it is not possible for a prophet to perish outside of Jerusalem. Therefore, what he says, I cast out demons, and accomplish healings today and tomorrow, and on the third day I am perfected (Luke XIII), is referred to his body which is the Church. …

On the Gospel of Luke ↗

Bonaventure

Thirdly, the passion of Christ is intimated as infallibly preordained from eternity; whence he adds: Nevertheless, I must walk today and tomorrow and the following day. This necessity was not from compulsion, but from a supernal disposition, according to that passage below in the last chapter: "O foolish and slow of heart to believe in all things which the Prophets have spoken," etc. …

Commentary on Luke, Chapter 13 ↗