Was it because the feast of unleavened bread was on the fifteenth day of the month?
"A Budget of Paradoxes, Volume I (of II)" by Augustus De Morgan
This was during the feast of the unleavened bread.
"The Children's Bible" by Henry A. Sherman
Spiritually to keep the feast of unleavened bread means to live in the energy of the new nature, walking in the Spirit.
"Studies in Prophecy" by Arno C. Gaebelein
AND the feast of unleavened bread, which is called the Passover, was at hand.
"A Translation of the New Testament from the original Greek" by T. Haweis
So the children of Israel which were present at that time held the passover, and the feast of unleavened bread seven days.
"The Bible Story" by Rev. Newton Marshall Hall
Passover is used in the general sense of the feast of unleavened bread, as Luke explains.
"A Harmony of the Gospels for Students of the Life of Christ" by Archibald Thomas Robertson
To walk in the energy of this nature is, in reality, to "keep" the feast of unleavened bread.
"Notes on the book of Exodus" by C. H. (Charles Henry) Mackintosh
These were to be the two grand characteristics of the feast of unleavened bread.
"Notes on the Book of Leviticus" by C. H. Mackintosh