There may be better ways, but one solution is:
A1: Date with month in question
A2: =date(year(a1),month(a1),1) [First day in month]
A3: =date(year(a1),month(a1)+1,0) [Last day in month]
A4: =a2+choose(mod(a2,7)+1,2,1,0,0,0,0,0) [First working day of month]
A5: =a3-choose(mod(a2,7)+1,1,2,0,0,0,0,0) [Last working day of month}
A6: =text(a4,"dd/mm/yy")&" - "&text(a5,"dd/mm/yy")