Septimanal

[Grogie]

Weekly. Occurring weekly. ''Going to church every Sunday is a septimanal activity.''

[Perry Lassiter]

Septi-or septim is obvious, but what does the suffix mean?

[Slava]

a suffix with the general sense “of the kind of, pertaining to, having the form or character of” that named by the stem, occurring in loanwords from Latin (autumnal; natural; pastoral), and productive in English on the Latin model, usually with bases of Latin origin (accidental; seasonal; tribal). Originally, -al1 was restricted to stems not containing an -l- (cf. -ar1.); recent lapses in this rule have produced semantically distinct pairs, as familiar and familial.

I figured the meaning was in the -al, so I looked up diurnal, and Lo! there it was.