Nè di venere nè di marte, non si sposa nè si parte; nè si dà principio all’arte

It is an ancient proverb, related to the superstition that Venus / Friday is a female divinity brings idleness and lust, while Mars / Tuesday is linked to war so it is better not to do anything else. Anyway, it can be an excuse if an activity fails, blaming the day it began, rather than a personal failure that could instead leads to a usefull self-critique.