It's in the afternoon on Sunday.和It's on the afternoon of Su

第二句正确,如果是表示“在下午”用 in the afternoon.如果表示“在某一天下午”或“在一个...的下午”用 介词on,如on a cold afternoon (在一个寒冷的下午)
该句还可以说成 It's on Sunday afternoom.