That is finishing early one day a month (4)
I believe the answer is:
ides
'one day a month' is the definition.
(Roman term for the middle day of a month)
'that is finishing early' is the wordplay.
'that is' becomes 'id est' ('that is' in Latin).
'finishing early' means to remove the last letter.
'idest' with its last letter removed is 'IDES'.
(Other definitions for ides that I've seen before include "Mid-month date in ancient Rome" , "One da" , "ill-fated time" , "Julius Caesar died in these March days" , "Eg March 15" .)