intercalation

[ in-tur-kuh-ley-shuh n ]

/ ɪnˌtɜr kəˈleɪ ʃən /

the act of intercalating; insertion or interpolation, as in a series.

something that is intercalated; interpolation.

Origin of intercalation

First recorded in 1570–80, intercalation is from the Latin word intercalātiōn- (stem of intercalātiō). See intercalate, -ion

The periods of intercalation were at first every two years, then three, and lastly four, and eight.

Harper's New Monthly Magazine, Vol IV. No. XX. January, 1852. |Various
In the days of Numa it consisted of twelve lunar months, with a system of intercalation something like that of the Greeks.

Harper's New Monthly Magazine, Vol IV. No. XX. January, 1852. |Various
They divide the year into twelve months of thirty days, and add five days by intercalation.

Encyclopaedia Britannica, 11th Edition, Volume 2, Slice 4 |Various
Whenever the intercalation is made there must necessarily be a change in the dominical letter.

Our Calendar |George Nichols Packer

It was with a view to that object principally that each Grecian city arranged its calendar and its system of intercalation.

Plato and the Other Companions of Sokrates, 3rd ed. Volume IV (of 4) |George Grote