Ji, its rather smog (not fog) on the occasion of Bhogi, a festival before the Pongal Day. On this day, people are expected to "burn the old and replace with new." The idea of the saying is to burn the rubbish in your inner self and change yourself to get a new start as the Sun sets to change direction (i.e., Dakshinayan to Uttarayan), but as it always happen, people take it in the literal sense and burn everything from domestic refuse to even empty deo cans on roads, thereby polluting the now-fragile atmosphere even more.