the Jewish New Year festival, held on the first (and sometimes the second) day of Tishri (in September). It is marked by the blowing of the shofar, and begins the ten days of penitence culminating in Yom Kippur.
Hebrew, literally 'head (i.e. beginning) of the year'
Rosh Hashana in other Oxford dictionaries
Definition of Rosh Hashana in the US English dictionary