The Free On-line Dictionary of Computing (30 December 2018):

Grandfather, Father, Son
GFS

    (GFS) A backup rotation scheme in which a
   "grandfather" backup is performed on the first Monday of each
   month, a "father" backup is performed on every other Monday
   and a "son" backup is performed on every other day of the
   week.  Grandfather tapes are kept for a year, father tapes for
   a month and son tapes for a week.  The exact schedule (and
   thus the number of tapes required) may vary, as may the choice
   of full backup or incremental backup, but the idea is that
   it should be possible to restore versions of any file of
   different ages: e.g. yesterday's, last week's or last year's
   version.

   (2004-10-11)