There is a discussion in the Poskim if when the 9th of Av falls on Shabbos and we postpone the Ta'anis to Sunday, do we consider
a) the Sunday 10th of Av fast a תשלומין (replacement) for the 9th of Av.
b) or in such a case Chazal established the original fast day on the 10th and there is no 9th of Av.
There are at least 3 differences in Halacha
1) a) May not learn Torah on Shabbos b) You may learn
2) a) The week before Shabbos is שבוע שחל בו ת"ב b) There is no ששבת"ב
3) a) If became Bar Mitzvah on Sunday no need to fast b) Needs to fast