The Collaborative International Dictionary of English v.0.48:

Intercalate \In*ter"ca*late\, v. t. [imp. & p. p.
   Intercalated; p. pr. & vb. n. Intercalating.] [L.
   intercalatus, p. p. of intercalare to intercalate to
   intercalate; inter between + calare to call, proclaim. See
   Calendar.]
   [1913 Webster]
   1. (Chron.) To insert, as a day or other portion of time, in
      a calendar.
      [1913 Webster]

   2. To insert among others, as a verse in a stanza; specif.
      (Geol.), to introduce as a bed or stratum, between the
      layers of a regular series of rocks.
      [1913 Webster]

            Beds of fresh-water shells . . . are intercalated
            and interstratified with the shale.   --Mantell.
      [1913 Webster]

WordNet (r) 3.0 (2006):

intercalate
    v 1: insert (days) in a calendar