What: It is the birthday of the universe, the day G‑d created Adam and Eve, and it’s celebrated as the head of the Jewish year. When: The first two days of the Jewish new year, Tishrei 1 and 2, beginning at sundown on the eve of Tishrei 1. Rosh Hashanah 2019 begins at sundown on September 29 and continues through nightfall on October 1 (see more details here).
https://www.chabad.org/library/article_cdo/aid/4762/jewish/What-Is-Rosh-Hashanah.htm