[neocon]

That's very likely, especially as Baptism is a symbolic drowning - a death to the life of world - followed by a rising to new life in Christ.

As I understand it, certain churches (Romanesque?) built to house important relics were also commonly octagonal in design.

The chapel at Mt. Savior Monastery in Elmira, NY is designed as cruciform (equal-arm) with an octagonal transept, deliberately, to bring to mind the Eighth "Day".

Finally, of course, the concept demonstrates that the early Church did not subscribe to a literal interpretation of Genesis, but that's a topic for another day.

[neocon]

Meant to add after the first sentence of #49: "Thus Baptism is seen as a transition from the Seventh to the Eighth 'Day'". Got to get some sleep....

[Romulus]

As I understand it, certain churches (Romanesque?) built to house important relics were also commonly octagonal in design.

Martyria were frequently of centralised design, and vice versa. Though this may have had something to do with a cultural memory of circular pagan tumuli, the centralised design also lent itself to theological interpretation, as you've already indicated.