You must be between 23.5° North Latitude (Tropic of Cancer) and 23.5° South Latitude for the sun to be at its zenith (Tropic of Capricorn). Except at the extreme boundaries, the Sun will be at its zenith two days every year, except on June 21 for the Tropic of Cancer and December 21 for the Tropic of Capricorn. It is called the Summer Solstice in both cases.
The position of the Sun at midday during the summer solstice determines how far north or south one can live within the northern hemisphere. If the Sun is more than half way across the sky then one can only live as far north as the Arctic Circle. If it is less than half way across the sky then one can live as far south as the Antarctic Circle.
The fact that the Sun will be at its zenith twice every year means that there are two opportunities each year when it is possible to see it directly overhead. The only times you won't be able to see the Sun directly are during a total solar eclipse when it blocks out all direct sunlight and enters a new Moon phase, or when it is completely obscured by clouds or dust particles in the atmosphere.
During a total lunar eclipse, the Earth's shadow falls on the moon causing it to darken around the edge that is facing towards the Earth. Because the moon is not fully illuminated by the Sun, only the parts that are shaded by the Earth will be affected by darkness.
The Sun reaches the observer's zenith when it is 90 degrees above the horizon, which occurs exclusively between the Tropics of Cancer and Capricorn. Elsewhere on Earth there are two other possibilities: the Sun could be directly overhead if you're standing on the equator or it could be due south (or north) if you're somewhere near the poles.
The location where this happens is called the Sun's zenith. All around the world except at the poles, the Sun will set once every day. At these locations night follows day instead.
Some places have more than one possibility for reaching maximum daylight hours during the year. For example, if the summer solstice falls on a Sunday in Arizona then the Sun will be directly over the center of Tucson at its highest point in the sky on the following Saturday too. The same is true for Santa Cruz, California; Bend, Oregon; and many other cities and towns across America.
Other locations have only one possibility for reaching maximum daylight hours each year. For example, in Toronto there is no part of the year when the Sun is due south of the city at sunset. However, because Toronto is located within Canada's temperate zone, there is also no season when the Sun isn't rising some time before sunrise and setting some time after sunset.
The sun will be directly above twice. The first time will be many weeks before the summer solstice, or June 21, when the vertical sun rays migrate north and reach the tropic of Capricorn, which is 23.5 degrees north latitude. From there they arc back south until they cross the equator on December 22. At this time it will be directly over the southern hemisphere.
The second time will be months after the winter solstice, or March 21, when the rays return south again until they cross the equator for the second time in December. At this time it will be directly over the northern hemisphere.
So, if you live at a latitude of 20 degrees north, you'll see the sun rise every day in April or May and set every night in October or November. It's out only during the remaining two months of the year.
At a latitude of 0 degrees north, where the Arctic and Antarctic circles meet, sun rises are confined to the time between April and September, when the sun is not too low in the sky. During these months you can see daylight all day long. But since sunset is followed by darkness again, the days don't seem to end.
At a latitude of 40 degrees north, where the equator lies, the seasons are reversed compared to lower latitudes.
The Earth's North Pole is at its most tilted toward the sun on the June solstice. The sun is directly overhead at 23.5 degrees north latitude, along the Tropic of Cancer, at this hour. The South Pole is at its most tilted away from the sun on the December solstice. The sun is directly over head at 23.5 degrees south latitude, along the Tropic of Capricorn.
The Earth's axis is not exactly perpendicular to its orbit around the sun. It makes a small angle called "insolation" with respect to the sun. This angle changes over time due to earth's changing distance from the sun (as it orbits around it) as well as its orientation relative to the sun during its year-long rotation.
On the summer solstice, the sun is at its highest altitude above the horizon throughout the day. From then until the next summer solstice, it will be getting lower and lower in the sky. On the winter solstice, the sun is at its lowest altitude below the horizon throughout the day. From then until the next winter solstice, it will be getting higher and higher in the sky.
The solar altitude is 90 degrees when the sun is directly overhead. At the equator, this happens during the vernal and autumnal equinoxes. The sun will be 90 degrees above the equator at the Tropics of Cancer and Capricorn on their respective summer solstices. It will be between 69 and 96 degrees above the equator at other times of the year.
At the poles, the solar altitude is 0 degrees for most of the year. However, around the spring and fall equinoxes the solar altitude is close to 90 degrees because the Earth is facing the Sun at a right angle. During these periods, the solar elevation is > 85 degrees at both the North and South Poles.
In addition, there are two more important dates in the solar calendar when the solar altitude is exactly 90 degrees: New Year's Day and Mardi Gras. These events usually take place near the tropics where they can be experienced by most people.
New Year's Day occurs on the first day of the first month after the winter solstice. This date varies from year to year because months have different lengths. But it always falls within a few days of January 1st.
Mardi Gras takes place several weeks before Easter. In Europe, it's celebrated on the Tuesday before Ash Wednesday. In Africa, it's held on a Sunday in February or March.