After reading all the posts few explanations from my knowledge as an electrical engineer....
People are not concerned about such switch off on other days, but worried now because we are in lockdown and hence industrial and railway loads are almost 0 now. The plants and grids are already operating in low load. When further load drop happens it might cause trouble. That's what everyone is worried about.
This activity is just a token of solidarity. To show the unity in diversity. Sheeps or humans, solidarity matters. Let's not relate this to any religious or astrological reasons. This is just solidarity. Together we can. That's it. No politics, no religion, no astrology.
Coming to original query, the power grid will not collapse. They will have many alternative methods to balance loads - like capacitor banks, reactors etc. Also grids can distribute the power to other parts of the grid which is not electrically stressed by creating alternating paths between grids. The circuit breakers placed in various sub stations can be utilised for this.
And not all the 1.2b Indians gonna follow this. So the actual drop will not be as big as expected. Just the grids and plants are put on alert to handle if such situation arises. And they can absolutely handle it. Nothing to panic. And even if we consider a grid failure, it's not that all the grids gonna fail altogether. That will never happen. There are many safety measures before that. Right from the circuit breakers on your house to breakers on the poles to transformers to sub stations.
Switch off only the lights and that too in phased manner. Everything and everyone will be safe.
People started thinking too much as they miss the fast foods which keeps them dull.
Just sit back and relax. Don't think too much.