The word day is required by the gender of the previous word first.  We might say "first of the month," or "first day of the month," and we would mean the same thing, only in the former case of "first of the month" we would only imply that we were talking about a "day."  The Greek, on the other hand, not only implies this.  It requires it.  For the gender of first is feminine, and the gender of the rest of the phrase (twn sabbatwn) is neuter.  The Greek adjective mia (mia, first) thus requires a word to agree with in gender.  That word is hmera (day).  The Greek could have used a neuter form for first to agree with the rest of the phrase.  That it did not shows that day is implied.  Back.