How often do they happen? When is the next one going to hapen?
A solar eclipse happens when the earth, the moon and the sun are on a straight line to each other, and the moon is between the earth and the sun.
Then the moon covers the sun, either partly or totally and that casts a shadow on the earth. If you are on the earths surface within that shadow, you can observe the solar eclipse.
The NASA has got a web page that has data about past and future solar eclipses. You can look up the frequency and location of solar eclipses there: http://funadvice.com/r/15nqoauln03
they occur because if people's sins and bad deeds when god want to make warning