Google announced on the morning of September 24, 2019: "Later today, we are releasing a broad core algorithm update, as we do several times per year. It is called the September 2019 Core Update." Rollout began at 2pm ET the same day, the second consecutive core update Google pre-announced. Danny Sullivan said it would take several days to reach all data centers, and Google did not announce a completion date.
Early readings pegged it as gentler than its predecessor. In Search Engine Land's data-provider roundup, RankRanger said volatility in positions 1-3 was substantially lower than during the June update, Sistrix reported "less of an impact compared to the June core update" with some clear winners but no significant losers identified, and Semrush saw no strong winner-loser pattern.
Being a broad core update, it carried no specific fix. If your rankings moved in late September 2019, review the affected pages against Google's core updates documentation and its self-assessment guidance, and follow our recovery guide.