This saying is born of long experience that if one release all week is going to break, it'll be the one someone rushed out on friday ;) No matter how you slice it, pushing new code always has a level of risk.
A sort of gold-plated ideal of CI/CD is the ability to push to prod any time. It's important that engineering have the confidence that they could push to prod any time, but less important to actually do so.
The reality is users don't give a crap if you don't push to prod every day; and in most cases they won't even know or care if you release on a slower cadence. Also you often have marketing/support/product management reasons to choose exactly when and how often to release new features to customers. Your support team will not thank you for releasing new features during their busiest period of the week; and marketing won't be happy if you release something right after they've done the weekly email without mentioning it.
I suspect many companies hit this balance:
So, am I afraid to push on fridays? No. Would I choose to push on fridays without a compelling reason? No.