THIS EPISODE IS SATURDAY'S DAF. What happens when Tisha B'Av falls out on Friday (which cannot happen in the era of a fixed calendar)? Namely, nobody should enter Shabbat in a state of affliction. But how can one eat on a fast day?! Note also that we do not have public fast days on Rosh Chodesh or Chanukah or Purim, with exceptions that don't establish a fast on one of the happy days. And then some rabbinic intrigue in terms of whose p'sak to follow - when Rabban Gamliel was alive, but does his p'sak stand when he's no longer alive? R. Yehoshua is willing to say no (granted, their disputes are well-known). Rigging the calendar made all of these difficult-to-navigate scenarios moot. Plus: In the new perek, the new mishnah, and when people went out of the Shabbat boundary, perhaps at the hands of non-Jews or one who forgot himself. When the sages ended up in a ship that was traveling on Shabbat, the ship went beyond techum, but the question was how much walking could they therefore do on the ship outside of the techum? Some walked the length of the ship and some stuck to 4 amot. Note that these cases might have been helped with an eruv.