A day the normal working week runs — which each statute defines for its own purposes.
A working day is a day that is neither a rest day nor a holiday. The definition sounds obvious and is not: what counts depends on which statute is asking.
A contract counting working days for a notice period, a court counting them for a filing deadline, and a payroll system counting them for a cut-off may each exclude a different set. Some exclude only public holidays; some exclude bank holidays specifically; some exclude half-days and some count them whole.
The practical rule is that "working day" is never portable between instruments. Where a document uses the term, it should say which days it excludes, and where it does not, that is a question and not a detail.
They are frequently synonyms. Where they are not, business day usually means a day banks and markets operate, which excludes days an office might well be working.
A working day is a unit of counting. Clear days is a rule about which end of the count you include.
See also business day · rest day · non working day · clear days · half day
We hold this as derived: any day not carrying a closure and not a rest day. See what we hold.