The Heptameron is short and it is organised as a timetable. Each day of the week has its angel, its planet, its colour, its perfume, its sigil and its conjuration, and the day is further divided into planetary hours each with its own ruling angel. The text supplies the circle to be drawn, the words to be said, and the correspondences to be observed, for each of the seven days in turn.
Its practical value is exactly that completeness. Where the Three Books of Occult Philosophy explain why angelic magic should work, the Heptameron tells an operator what to do on a Tuesday. That is why it travelled bound with the pseudo Agrippa Fourth Book and why the two are often treated as one work: between them they supply the missing operative layer of a philosophical system.
The angels named here, and the days and hours scheme, pass more or less unchanged into the Golden Dawn material and from there into modern ceremonial practice, so this small text has an influence out of proportion to its length.
The attribution to Pietro d'Abano, a fourteenth century physician and astrologer condemned by the Inquisition, is not sustained by the manuscript evidence.
Its scheme of planetary hours, in which the hours of daylight and darkness are each divided into twelve unequal parts that lengthen and shorten with the season, is the same one used across medieval astrology, and it is the part of the book still in general use.