There is a strong tradition (maybe halakha?) to not engage in most learning on Tisha b'Av (with a few exceptions). Many of us learn daily and I understand that we generally suspend our learning for that one day (or choose to learn a permitted topic, like hilchot Tisha b'Av).
However, daf yomi - a page of Talmud every day - is more organized. People globally learn on the same schedule. And that schedule seems to include 9 Av. What gives? Why are we allowed to learn Pesachim 26 on 9 Av but almost nothing else?