Google opened 2020 with a same-day core update. On the morning of January 13, @searchliaison announced "Later today, we are releasing a broad core algorithm update, as we do several times per year. It is called the January 2020 Core Update", and rollout began around 12pm ET, followed by "The January 2020 Core Update is now live and will be rolling out to our various data centers over the coming days." It was the first core update since September 2019.
Three days later Google said the update was mostly done rolling out, while noting core updates can take up to two weeks to fully complete; no formal completion date was ever announced. Tracking tools including Mozcast, RankRanger and Semrush showed substantial ranking volatility during the rollout, and Search Engine Land's 2020 year in review summed it up as an update that "was big and impacted a lot of sites".
As with every broad core update, there was no penalty to lift and nothing specific to fix. If your site moved in mid-January 2020, assess the affected pages against Google's core updates documentation and its self-assessment guidance, and see our recovery guide for how recovery typically plays out.