Answer:
On the Cancer tropic (23.5° latitude north) it will happen once every year, on the day of the northern hemisphere solstice (about June 21st).
On the Capricorn tropic (23.5° latitude south) it will happen once every year, on the day of the southern hemisphere solstice (about December 21st).
On the equator it happen twice every year. One on each equinox (about March 21st and September 21st).
For any other given place between the tropics, it will also happen twice every year.
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