Well, I disliked dies in the feminine here because of the meaning of “a set day, appointed time, term” (in French “une date”, not “un jour”). On the other hand, singuli, ae, a is very rarely used in the singular.
Now see what Seneca wrote (Epistles, III, 26, 4) :
Non enim subito inpulsi ac prostrati sumus: carpimur, singuli dies aliquid subtrahunt
viribus." French translation :
“car nous ne sommes pas terrassés, anéantis d’un seul coup; minés insensiblement, nous voyons nos forces décroitre chaque jour”, so “(We don’t become weak at once, but) every day takes something from our strength”.
Here we have singuli dies as a subject in a slightly post-classical and slightly poetical Latin. I think it can be good.