Translation:She says yes or no, depending on the day.
"depending on the day" can be translated to "according to the day" But, where does "day" become pluralized into "les jours"?
"Selon le jour" can be found, but it often has a nuance: one day of the week or the other.
"Selon les jours" is broader in meaning, and here, it refers to her changing her opinion very often, which might not be a matter of her doing so on a Monday or a Tuesday.
There is another error Duo. please rectify. after research I found various examples ( not just one) on the web, where selon les jours can be translated into English in singular AND PLURAL so my translation depending on the days ( plural) or according to the days... is also correct and not incorrect Although I appreciate that I have learnt that selon les jours also can be used in singular... depending on the day...
The plural version "days" is non-idiomatic on English. We are translating idiom to idiom, so both must be respected. (The idiom means that she is unpredictable, she often changes her mind on a whim.)