The POEC has created and collected useful resource material on the Zero Shadow Day. For a given place below +23.5 degrees latitude in India, this occurs when the Sun is exactly overhead and hence casts no shadow on a vertical object. This occurs twice a year, once during Uttarayan and once during Dakshinayan.
For more information – maps, lists, tables, articles, etc, see
https://astron-soc.in/outreach/activities/zero-shadow-day/
