Zero shadow day is when the sun is at the highest point in the sky between the Tropic of Cancer and the Tropic of Capricorn. Now when the sun is just overhead, then the shadow of objects disappears.
This is exactly what happened at Bhuvneshwar, Odisha where this rare phenomenon occurred on 11:43 a.m. on 22 May 2021. It happened for a very short period of time, for about 3 minutes.
As mentioned by Deputy Director of Pathani Samanta Planetarium, Bhubaneswar, Subhendu Patnaik :
The phenomenon takes place as the rotation axis of the Earth is tilted at an angle of 23.5° to the axis of revolution around the Sun. When the location of the Sun moves from 23.5° N to 23.5° S of Earth’s equator, all places whose latitude equals the angle between the location of the Sun and the equator will experience Zero Shadow Days.
It was also stated by the officials that the following cities can experience Zero Shadow Day on the following date:
Sambalpur - May 28, 11.51 am and July 15, 12 pm
Burla - May 28, 11.52 am, July 15, 12 pm
Rourkela - June 2, 11.48 am, July 10, 11.56 am
Baripada May 31, 11.41 am, Juy 12, 11.49 am