Aim: We aimed to explore the organization of the calendar knowledge base underlying date calculation by assessing the ability of savant calendar calculators to free recall a series of date lists. Method: Four experiments are reported that assessed recall of structural and non-structural features of the calendar in eight savant calendar calculators (seven males; one female; median age 34y 6mo; age range 27-47y), five of whom had a diagnosis on the autism spectrum. The inclusion criterion was a genuine calculation ability rather than an interest in dates. Results: Mean recall was facilitated for material organized according to the structural features of a calendar (leap years, dates falling on the same weekday, dates occurring at 28 y intervals) but not for a non-structural calendar feature (Easter Sunday). Interpretation: Distinctions are drawn between two sources of savant calendar-related knowledge, structural and event related. It has been suggested that structural knowledge plays a key role in the acquisition and operation of savant date calculation skills.