When I have been in mainland Europe, in areas such as France or Belgium, a lot of the shops say they're open "7/7".
The latter 7 presumably means 'every day' like in 24/7, but what about the other 7?
Surely it's not uncommon to be open for 7 hours in a day?
In my eyes 7/7 is shorthand for French 7 jours sur 7 which translates to 7 days out of 7. Meaning the business is open all week. Note that this does not necessarily mean that the business is also open 24h.
Indeed in France you often read 7j/7 written on shops that are open every day of the week (note the little j, standing for jours == days). For example, take this Parisian info website showing a list of 24h businesses, and look at the description of the 24h pharmacy:
Pharmacie ouverte 24h/24 et 7j/7 à Paris