The same applies in spanish! tener un día de mierda, for example, means you are having a horrible day.Just as an observation; there is more than one sense to the idea of "crap or crappiest."
If the speaker intended vulgarity, "shit or shittiest" work well.
But wretchedly bad or miserable also work as an English synonym if the speaker intended to avoid vulgarity. In other words "crap" does not necessarily mean "excrement" it can just as easily mean something "bad or worthless."