In three weeks, the Fast of the 9th of Av will be on the 10th of Av.
Today’s fast day actually was supposed to take place on Saturday, which was the 17th day of the month of Tammuz, whose laws are reported here.
The Shabbat postpones the fast, leaving us with the seeming contradiction of marking the Fast of the 17th Day of Tammuz on the 18th day.
In three weeks, there will be a replay in three weeks, when the Fast of the Ninth Av will be noted on the 19th of Av for the same reason that one does not fast on Shabbat.
The exception to the rule is the fast of Yom Kippur, which takes precedence even if it falls on Shabbat.
Judaism is full of many apparent contradictions but which are logical after studying Jewish law
For example, it is forbidden to erect a tent, including umbrellas, on Shabbat. Jews also may not drive a car on Shabbat, unless it is a matter of life and death.
That is why you might see a Jew walking half a mile to synagogue in the winter in a wicked rainstorm and totally drenched when he arrives for prayers in which he asks God for rain.