"She says yes or no, depending on the day."
Translation:Elle dit oui ou non, selon les jours.
This is just a guess on my part (not a native speaker here) but it seems like the French have a different way of expressing multiple days. To do something each and every day as in a repeated habit - He runs every day - they use "les jours". Il court tous les jours.
I think it might be the same idea here where multiple days are involved - depending on the day - selon les jours. Only a guess - I couldn't find an definitive answer.
However I did find where selon les jours maybe translated as depending on the day, from day to day, per day, and according to the day. Check outr link: https://www.linguee.fr/francais-anglais/traduction/selon+les+jours.html
I am also a bit surprised that an administrator has not offer an answer to this question.