Impact on grid if entire country turns off all electrical appliances within few seconds and then turns them on within few seconds
The article explains it fairly well.


  In Uttar Pradesh, India’s most-populated state, the local utility plans to switch off electricity in phases to stop the surge in voltage from threatening the state’s power grid and equipment, U.P. Power Transmission Corp. said in a letter sent to generators. The country plans to switch off some of its hydropower generation capacity to deal with the sudden dip in demand, Sahai said. The power ministry, in a statement, said adequate arrangements had been made to handle the variation in demand.


While the load reduction will be rapid it will not be instantaneous. Most people will switch off their lights but leave fridges, pumps, and other loads running and I would be surprised if the load reduction was not spread over a couple of minutes.

As mentioned in the article, hydro can be switched off fairly quickly, solar will already be off so it will be a matter of venting steam on the rest of the generators to maintain the grid frequency. Presumably wind generation can be switched off as required.