Reach Altus Plateau
Rest at a Site of Grace in Altus Plateau
Defeat Starscourge Radahn
Talk to Ranni at Ranni’s Rise, then progress her quest until the festival is available
Talk to Blaidd in Siofra River, then return to Redmane Castle
Trigger the festival by entering Redmane Castle after meeting the conditions above
Alternatively, reach Altus Plateau and then use the Grand Lift of Dectus or Ruin-Strewn Precipice to activate the festival state