‘‘During the holiday season, we want to be considerate of all our players with the release timing of new content. Many guilds are still progressing through The Eternal Palace, and we want to give them enough time to finish the raid and earn their Cutting Edge: Queen Azshara and Ahead of the Curve achievements before opening the new raid. We try to avoid releasing new content tiers during the holiday season so that players are not encouraged to choose between progression and spending time with their families and friends. This also helps ensure us that we’re fully staffed to support any required hotfixes or turning issues that may emerge.’’
Source: Raiding Expectations for the Holiday Season
Blizzard in 2020: Oh mythic raid difficulty releases 10 days before Christmas btw