Celaya is the third largest city in the State of Guanajuato, after Leon and Guanajuato.
Coming from Guadalajara you can catch ETN the Executive class service with an option of single seats and WiFi with lunch for around M$350 (pesos) (US$25) one way or a lower class first class service from Primera Plus for 333 pesos where you get just a free soda and cookie.
From San Miguel de Allende there is a second-class Flecha Amarilla bus every 15 minutes for M$55 and a second-class Servicios Coordinados bus (with TV and air conditioning) for M$61 once an hour. Both stop in Comonfort and Escobedo on the way. There is also a less-frequent first-class Primera Plus bus.