John Gill Commentary Numbers 28:17

"And on the fifteenth day of this month shall be a feast: seven days shall unleavened bread be eaten." — Numbers 28:17 (ASV)

And in the fifteenth day of this month is the feast
Not of the passover, that was the day before, but of unleavened bread, which began on this day, and lasted seven days, (Leviticus 23:6) which is what the Jews call the Chagigah:

seven days shall unleavened bread be eaten ;
see (Exodus 12:15Exodus 12:18) .