Quiet hours
After-hours maintenance is useful for upgrades, cutovers, cabinet work, wireless changes and the delicate sort of tidying that is easier when nobody is trying to print, pay, consult or log in.
Why late?
When a team has left for the day, signal tests are clearer, patching is less disruptive, cabinets are accessible and nobody loses half an hour because a till is rebooting during a lunch queue.
A quieter mode of work
It is also, occasionally, the only time a building becomes cooperative enough to finish the awkward parts properly. No special ceremony, just good timing, measured steps and enough patience to let the space agree with what is being changed.