Calendar calculating in 'idiots savants': how do they do it?

A number of mentally handicapped individuals are able to solve difficult calendar date problems such as specifying the day of the week for a particular date, sometimes over spans of more than 100 years. These individuals are self-taught and do not follow procedures at all similar to the usual, published, algorithms. An investigation of one individual revealed that he retained considerable information about the structure of days in particular months, probably as visual images. His skill closely depended on the extent and form of his knowledge of calendars, and his errors were often a consequence of lack of knowledge about a particular time period. Mentally retarded individuals who perform calendar-date feats are often socially withdrawn and devote considerable periods of time to calendar dates. The most capable calendar-date calculators are usually individuals who have a strong interest in calendars as such.