why there is a circle of light surrounding Sun and Moon? One day when I was playing basketball I saw a circle around the bright Sun. It is a perfect circle with a radius of about 20-30 km. 

Why does this circle of light form? I have seen the same also around the moon. But this does not happen every day. Why ?
Most probable you have been experiencing a weather phenomena: Ice crystals in the sky reflect or bend the light to form a halo.

So, this depends on the temperatures and moisture content of the atmosphere and cannot be seen every day.