It is a good rule of thumb when visiting Greece, to ask more than one person for the same information, especially when asking people on the street. If at least three people agree on something, then there's a 50% chance that you've gotten correct information.
Outside of the cities it's a good precaution to ask more than one employee at bus stations for bus details, especially during such holiday periods as Christmas.
Bus schedules in smaller towns and villages during such vacation breaks are often different from the regular ones posted at the stations, many routes in such areas geared towards public school needs, since there are few other passengers. There may be far fewer buses during holiday breaks, with different departure times as well.