In the Piyut of Echod Mi Yodea it says Shemoinoh Yemei Milah
eight days of Milah (based on the gemoro Eruvin 40b)
The meforshim ask , There is only one day of Milah on
the eight day, and not eight days of Milah.
I heard an answer given by R. Chaim Kanievsky Shlita.
There is an obligation not to make a Bris before
the eight day.*
Therefore there are eight days of Milah .
The first seven days to make sure not to do
the Bris and on the eight day to do it.
*many poskim hold that you weren’t yotzei mitzvahs milah