intercalary
adjective
in·ter·ca·la·ry in-ˈtər-kə-ˌler-ē 
ˌin-tər-ˈka-lə-rē
1
a
: inserted in a calendar
an intercalary day
b
of a year : containing an intercalary period (such as a day or month)
2
: inserted between other things or parts : interpolated
Example Sentences
Recent Examples on the Web The reflection between Sarat's private ordeal and the country's vast, ongoing calamity is sustained by a series of intercalary chapters: excerpts from history books, news reports, memoirs and speeches. 
The Washington Post, OregonLive.com, 30 May 2017
The Washington Post, OregonLive.com, 30 May 2017 Word History
Etymology
borrowed from Latin intercalārius, from intercalāre "to intercalate" + -ārius -ary entry 2
First Known Use
1614, in the meaning defined at sense 1a