Numbers 33:3
"On the fifteenth day of the first month; on the morrow after the Passover..."
The 15th is distinguished from Passover itself.
Leviticus 23
14th = Passover
15th = Feast of Unleavened Bread
Two separate calendar designations.
Numbers 28
14th = Passover
15th = Feast
The Torah consistently distinguishes the Passover sacrifice from the feast day.
John 19:31
"For that Sabbath day was an high day."
Read as describing the day itself rather than necessarily the following day.