While the option for the procession certainly exists for Palm Sunday, here is what is supposed to happen:
- The commemoration of the entrance of the Lord into Jerusalem has, according to ancient custom, been celebrated with a solemn procession, in which the faithful in song and gesture imitate the Hebrew children who went to meet the Lord, singing “Hosanna.” 
The procession may take place only once, before the Mass that has the largest attendance, even if this should be in the evening of either Saturday or Sunday. The congregation should assemble in a secondary church or chapel or in some other suitable place distinct from the church to which the procession will move.
In this procession, the faithful carry palm or other branches. The priest and the ministers, also carrying branches, precede the people. 
The palms or branches are blessed so that they can be carried in the procession. The palms should be taken home, where they will serve as a reminder of the victory of Christ, which they celebrated in the procession.
Pastors should make every effort to ensure that this procession in honor of Christ the King be so prepared and celebrated that it is of great spiritual significance in the life of the faithful.
The Missal, in order to commemorate the entrance of the Lord into Jerusalem, in addition to the solemn procession described above, gives two other forms, not simply for convenience, but to provide for those situations when it will not be possible to have the procession.
The second form is that of a solemn entrance, when the procession cannot take place outside of the church. The third form is a simple entrance such as is used at all Masses on this Sunday that do not have the solemn entrance. 
- Where the Mass cannot be celebrated, there should be a celebration of the word of God on the theme of the Lord’s messianic entrance and passion, either on Saturday evening or on Sunday at a convenient time. 
- During the procession, the choir and people should sing the chants proposed in the Roman Missal, especially Psalms 23 and 46, as well as other appropriate songs in honor of Christ the King.
- The passion narrative occupies a special place. It should be sung or read in the traditional way, that is, by three persons who take the part of Christ, the narrator, and the people. The passion is proclaimed by deacons or priests, or by lay readers. In the latter case, the part of the Christ should be reserved to the priest.
The proclamation of the passion should be without candles and incense; the greeting and the sings of the cross are omitted; and only a deacon asks for the blessing, as he does before the Gospel.  For the spiritual good of the faithful, the passion should be proclaimed in its entirety, and the readings that proceed it should not be omitted.
- After the passion has been proclaimed, a homily is to be given.
The procession does not occur during the middle of the Mass; rather, it occurs at the beginning. The proclamation of the Passion occurs in the middle and occupies a significant chunk of the liturgy.