A shofar or ram's horn is used to summon Jews to Rosh Hashanah services. A reference to this holiday from Leviticus 23:24-25 mentions the horn, but not the name of the holiday:

24: Speak unto the children of Israel, saying, In the seventh month, in the first day of the month, shall ye have a sabbath, a memorial of blowing of trumpets, an holy convocation.

Rosh Hashanah, the Jewish Year, falls on the first and second days of Tishri. This year in the Jewish Calendar is 5768. Rosh Hashanah runs from sunset September 12, 2007 to nightfall September 14, 2007
2 Jewish New Years - Rosh Hashanah and Passover
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Dates of the Holidays for the Jewish Year 5768

Shofar image © Clipart.com