POETS Day is a term used in the Army to refer to Friday as the last day of the work week, not (as understood by your average Student Grant) as the Day of the week set aside for sitting by the River Cam, Thames, Nile, Mississippi with your sweetheart balanced on your knee and spouting shakespearian or Wordsworth or Whatever drivel at her whilst trying desperately to get inside her underwear!.
The letters in "POETS" are an acronym for "Piss Off Early Tomorrow's Saturday": hence Friday becomes a Poet's day. Variations on this are:
"Punch Out Early Tomorrow's Saturday" (referring to a manual punch time clock),
"Push Off Early Tomorrow's Saturday" and
"Push Off Early Tomorrow's Sunday" (based on the old 6 day work week).