Bungie announced all the details that we need for the race in Grandmaster Nightfalls, including the rules for the first week, requirements and the start date.
Grandmaster Nightfalls will kick off on the next weekly reset which will take place on March 16, 2021, at 18:00 GMT / 10:00 AM PT. Contest mode will be enabled initially, following the ruleset that is mostly the same in Raid races.
That means that the minimum power required will be 1325 and will be the only power level for the first week since high power levels will be downscaled.
Each Grandmaster Nightfall will have a different set of modifiers and one GM Nightfall will be available per week, until April 20, 2021. This is when all of them will become available and the farm fest will begin for the Guardians.
All of the GM Nightfalls will be then available until the end of the season and if you missed some during their weeks, you will have the opportunity to catch up then.
Keep in mind that GMs will be awarding Adept weapons for those who successfully complete the challenge. Additionally, the gilded version of Conqueror will start appearing.