The sun can be seen directly above oneself twice a year if you live between the tropic of cancer and the tropic of Capricorn.
The Tropic of Cancer and the Tropic of Capricorn each lie at 23.5 degrees latitude. The Tropic of Cancer is located at 23.5° North of the equator and runs through Mexico, the Bahama, Egypt, Saudi Arabia, India, and southern China. The Tropic of Capricorn lies at 23.5° South of the equator and runs through Australia, Chile, southern Brazil (Brazil is the only country that passes through both the equator and a tropic), and northern South Africa.
The zenith time is when the sun at its zenith is directly above us. It changes depending on which latitude you are on the ancient Egyptian and Greeks found this out and used it to figure out what latitude they were on. We have used it in a similar fashion.
For us the crucial dates are :
March 1. and October 13. each year.
These are the times when looking at the shadow of a perpendicular pole the shadow completely disappears at the time of the zenith. This is illustrated in the pictures below.