The sense of day used here is the day's work on the field of battle.
RELATED IDIOMS :
all in a day's work
(of something unusual or problematic) accepted as part of someone's normal routine or as a matter of course.
at the end of the day
when everything is taken into consideration - British informal
1995 - Jayne Miller - Voxpop Today - I've been giving out leaflets. You don't have to, but at the end of the day, it's worth it.
call it a day
decide or agree to stop doing something, either temporarily or permanently.
This expression comes from the idea of having done a day's work. In the mid 19th century, the form was call it half a day.
day in day out
continuously or repeatedly over a long period of time
day of reckoning
the time when past mistakes or misdeeds must be punished or paid for
a testing time when the degree of your success or failure will be revealed
This expression refers to the Day of Judgement, on which, according to Christian tradition, human beings will have to answer to God for their transgressions.
do not give up the day job
used as a humorous way of recommending someone not to pursue an alternative career at which they are unlikely to be successful – informal
1996 - Charlie Higson - Getting Rid of Mr. Kitchen - 'You are the worst beggar I have ever encountered,' I said. 'Don't give up the day job.'
from day one
from the very beginning
1996 - Christopher Brookmyre - Quite Ugly One Morning - The system churns out junior doctors who have paid bugger-all attention to the meat and two veg medicine they will find themselves up to their necks in from day one.
have had your day = have had its day
be no longer popular, successful or influential
if he is a day = if she is a day
at least (added to a statement about the age of a person or thing)
1992 - Shashi Tharoor - Show Business – Lawrence must be fifty if he's a day.
just another day at the office
1997 - Times - Professional cricket has been reduced to just another day at the office.
make a day of it
devote a whole day (or night) to an activity, typically an enjoyable one.
make his day
make an otherwise ordinary or dull day pleasingly memorable for someone
not his day
used to convey that someone has suffered a day of successive misfortunes – informal
1997 - A. Sivanandan - When Memory Dies – He sighed inwardly, this was not his day.
one of those days
a day when several things go wrong
a red letter day
a pleasantly memorable, fortunate or happy day
In Church calendars, a saint's day or church festival was traditionally distinguished by being written in red letters.
seen better days = known better days
be in a worse state than in the past
have become old, worn-out or shabby
that will be the day
something is very unlikely to happen – informal
1991 - Alistair Campbell - Sidewinder - Now for my proposal, which you'll find irresistible.' "That'll be the day.'
those were the days
used to assert that a particular past time was better in comparison with the present
1997 - Brenda Clough - How like a God - ‘Those were the days,' Rob said. 'B.C. - before children! Remember?'