Between the two tropic lines it can do so on two days each year as the Sun moves between the southern and northern hemispheres of the sky, and outside of the tropics the Sun can never reach the zenith.
At 40 degrees south latitude the Sun's maximum altitude in the sky is displaced by 40 degrees to the north in the sky compared with the values at the equator and at 40 degrees north latitude it is displaced by 40 degrees to the south, giving a lower and upper limit of midday altitude of the Sun through the year at 40 degrees latitude in each hemisphere of 26.5 degrees and 73.5 degrees.
See the lower chart on here, measured at latitude 42 degrees north
http://www.griffithobs.org/skyfiles/skys… . . . . .
http://imagine.gsfc.nasa.gov/docs/ask_as… . . ..