Numbers 29:1 "It shall be a day of blowing the Shofar for you"
Rosh HaShanah is the beginning of the new year.
On this day, we proclaim HaShem to be King over the entire
Universe.
Just as a king's arrival is announced by trumpet blowing, we
sound the shofar on Rosh HaShanah, declaring HaShem the one and
only King.