This should have been easy to look up, but not quite so! We will travel from Florence to San Gimignano on Sunday October 9th. Looks like a Sunday Schedule may be different than weekday schedule. Not sure of the bus schedule?
Here is a link to the bus schedule you want. It is the SITA bus line and the station is close to the main train station in Florence. http://www.sitabus.it/sita-toscana/Firenze-S%20Gimignano2010.pdf The route is Florence to Poggibonsi, change to another bus and then Poggibonsi to San G. the schedules are on page 2 on the right. Above the each departure/arrival pair of times is a key symbol. The one you want looks like a small Christian cross, that means it runs on Sundays and holidays. If there is no symbol at all, it should mean it runs every day.