Google announced on 3 Dec that it would be releasing a broad core algorithm update - the December 2020 Core Update - later the same day.
In a follow up tweet a few hours later, it confirmed that the rollout had started.
Google typically releases a core update every few months, and the last one was in May.
RankRanger is showing large flucutations on 4 Dec for both desktop and mobile SERPs.
Google's advice to sites hit by a core update can be seen here.
We'll be updating our blog as details emerge about the impacts of the core update.