Chazal (our sages) have addressed and unstuck this "potential sticking
point." Unfortunately, I am at my office as I write this, so I do not have
the seforim (books) I need to give precise references. However, Midrash
Rabba addresses the question of why after describing what was created on
the sixth "Yom", the Torah says "Vayehi erev vayehi voker, yom hashishi"
(It was evening, it was morning, the sixth day), whereas after each of the
first five days this formula appears without the definite article ("ha",
which means "the"). The answer (or at least one of the answers) Midrash
Rabba gives is that until that point the Torah was referring to "days" of
creation, but from then on it would be referring to regular days. (This is
not an exact translation, because I cannot recall the exact language of the
Midrash Rabba. Anyone interested, however, can find it by looking at Rashi
on the phrase "yom hashishi", because he quotes the Midrash Rabba.)
Thus, according to this Midrash Rabba, the first five "days" of creation
(or six, depending upon how you understand the Midrash) were different from
the seventh (or six and seventh). The seventh "yom", which was the first
Shabbos, was 24 hours, just like every Shabbos since then.
Jeff Zuckerman