An Etymological Dictionary of Astronomy and Astrophysics

To insert (an extra day, month, etc.) in the calendar to make it follow the seasons or moon phases.

Etymology (EN): From L. intercalatus p.p. of intercalare “to proclaim the insertion of an intercalary day or month,”
from → inter- + calare “to proclaim, announce solemnly;” → calendar.

Etymology (PE): Andarhelidan, andarheštan “to insert,” from andar-→ inter- + helidan, heštan “to place, put” from Mid.Pers. hištan, hilidan “to let, set, leave, abandon,” Parthian Mid.Pers. hyrz; O.Pers. hard- “to send forth,” ava.hard- “to abandon;”
Av. harəz- “to discharge, send out; to filter;” hərəzaiti “releases, shoots;” cf. Skt. srj- “to let go or fly, throw, cast, emit, put forth;” Pali sajati “to let loose, send forth.”