Before 1970, the season of Lent had a slightly different structure than it does now. If you happen to look at a missal from back then, you'll notice that there are only four Sundays of Lent, followed by Passion Sunday, followed by Palm Sunday. In a current missal you will typically see that Palm Sunday is now labeled “Passion (Palm) Sunday”.
Why is this? What did a separate day signify before 1970?
According to Dom Gueranger in The Liturgical Year, “This Sunday is called Passion Sunday, because the Church begins, on this day, to make the sufferings of our Redeemer her chief thought”. Traditionally, all statues and crucifixes were veiled at the Vespers for Passion Sunday. The Introit, Gradual and Tract all are petitions to save the just from the persecution of the unjust and the Tract even foreshadows the scourging.
At that time Jesus said to the multitudes of the Jews: Which of you shall convince Me of sin? If I say the truth to you, why do you not believe Me? He that is of God heareth the words of God. Therefore you hear them not, because you are not of God. The Jews therefore answered, and said to Him: Do not we say well, that Thou art a Samaritan, and hast a devil? Jesus answered: I have not a devil: but I honor My Father, and you have dishonored Me. But I seek not My own glory: there is One that seeketh and judgeth. Amen, amen, I say to you: If any man keep My word, he shall not see death for ever. The Jews therefore said: Now we know that Thou hast a devil. Abraham is dead, and the prophets: and Thou sayest: If any man keep My word, he shall not taste death for ever. Art Thou greater than our father Abraham, who is dead? and the prophets are dead. Whom dost Thou make Thyself? Jesus answered: If I glorify Myself, My glory is nothing: it is My Father that glorifieth Me, of whom you say that He is your God, and you have not known Him: but I know Him: and if I shall say that I know Him not, I shall be like to you, a liar. But I do know Him, and do keep His word. Abraham your father rejoiced that he might see My day: he saw it and was glad. The Jews therefore said to Him: Thou art not yet fifty years old: and hast Thou seen Abraham? Jesus said to them: Amen, amen, I say to you, before Abraham was made, I am. They took up stones therefore to cast at Him: but Jesus hid Himself, and went out of the temple.