Wednesday night marks the beginning of Rosh Hashanah, one of the Jewish High Holidays. Rosh Hashanah begins at sundown and lasts for two full days after tonight. It also marks the start of the Jewish holiday season and includes a number of festive traditions. Here is what you need to know about the Jewish holiday of Rosh Hashanah. When is Rosh Hashanah and what time does it start? Rosh Hashanah 2017 starts at sundown on Wednesday, Sept.