The Collaborative International Dictionary of English v.0.48:

Term day \Term day\
   A day which is a term (as for payment of rent), or is a day
   in a term, as of the sitting of a court; esp., one of a
   series of special days, designated by scientists of different
   nations or stations, for making synoptic magnetic,
   meteorological, or other physical observations.
   [Webster 1913 Suppl.]