Bus Schedule from Siena to Florence on SUNDAY (Sept 14, 2014)

There are not many on Sundays. I was there last month. I took the 9:40 (approximately) bus back to Florence. I went the day prior and asked what time the buses were. I think there was another one mid afternoon. Someone had sent me a schedule but it did not match what I was told.

tsurutuenge,
See http://www.busfox.com/timetable/

If the bus schedule on Sunday is too limited for your needs you can always take the train. It's not much longer (1.5 hours, just 10 min more than the bus) and the train station in Siena is at the bottom of the hill. Trains on Sundays are way more frequent than the bus (at least one per hour in each direction) and they have more extended schedule. The first train on a Sunday departs from Florence at 6:20 and the last one back departs from Siena after 9:30pm.