BSE and NSE conduct a special trading session for an hour on the day of Diwali. This trading session is known as Muhurat trading. The time of the 1 hrs muhurat trading session changes every year as it is based on the Diwali Pooja Muhurat.
Exchanges announce the Muhurat trading timings a few days before Diwali in a circular/notice. Usually, it's in the evenings between 4 to 7 PM on the day of Diwali.