Doing a quick search on google more or less reaffirmed what they taught me in RCIA: Christians associated 8 with rebirth and the resurrection because Christ rose on the eighth day. Since baptism is a participation in his death and resurrection and results in our rebirth, it was common to make baptismal fonts and baptistries in the shape of an octagon reflecting this.
Or … as its often counted, the “8th day of the week”
He rose on the 3rd day after his crucifixion… which was the 6th day of the week. The next day was a Sabbath (and therefore the 7th day) and he rose on the day after that (the 8th day of the week… otherwise known as the first day of the week).
Early Christians saw this to be significant, especially regarding it being a “new Creation” because of its paralells/differences with the Genesis 1 account.
DISCLAIMER: Catholic Answers has turned over the archive to Catholic-Questions.org and no longer owns, manages, or moderates the forums. For additional apologetics resources please visit www.catholic.com.