The blessing is called "Blessing on the Sun" or "Birkat Hachama" in Hebrew.
The Sun makes a 28 year cycle to return to the position that it was in when the Universe was first created. The blessing is recited at this point in time, once every 28 years.
The text of the blessing is: "Blessed are You, Lord, our God, King of the Universe who makes the works of Creation."
The same blessing is recited upon experiencing numerous natural phenomena such as lightning, comets, meteor showers and certain amazing natural wonders.
There is great excitement associated with this event since it is so rare. Very large crowds gather together to recite it at certain places around the world; in particular at the Western Wall in Jerusalem.
Don't miss it because you will need to wait another 28 years for another chance!