What does "optional" mean?

Whilst I was working on the Sequence for the Feast of the Body and Blood of Christ, it was pointed out to me that this sentence from the GIRM, which I and all the musicians I know thought pretty straightforward, is nothing of the sort, it is very ambiguous...

"The Sequence, which is optional except on Easter Sunday and on Pentecost Day, is sung before the Alleluia."

I was naturally reading "except for on..." to mean "except for on... when it is required, DUH," and although the sheer existence of Victimae Paschali and Veni Sancte would indicated that this is the correct reading, it was pointed out that the sentence on its own could mean "except on.... when it is forbidden."