The challenge, should you decide to accept it: use one of these two words in a sentence that sheds some light on their meanings.
Intercalarity – The burden of the song; the putting between, as the burden is between the verses.
Intercalary – adj. 1) Of a day, days, or month: Inserted at intervals in the calendar in order to bring an inexact reckoning of the year into harmony with the solar year. 2) Of a line or stanza: Inserted at intervals in a composition; of the nature of a refrain. 3) Of the nature of an insertion between the original or ordinary members of a series or parts of a whole; interpolated, intervening.