Rosh Hashanah is the beginning of the Jewish New Year, in this case, 5786, and kicks off the High Holy Days. It is an important two-day holiday that focuses on long prayer services in synagogue, spending time with family, and praying for repentance as God judges you for the year and writes his judgment in the Book of Life.
This year, Rosh Hashanah starts on the evening of September 22 and ends on the evening of September 24. The next day, the third day of Tishrei, is the fast day known as the Fast of Gedaliah.