Counting of the Omer (or Sefirat Ha'omer, Hebrew: ספירת העומר) is a verbal counting of each of the forty-nine days between the Jewish holidays of Passover and Shavuot. This mitzvah derives from the Torah commandment to count forty-nine days beginning from the day on which the Omer, a sacrifice containing an omer-measure of barley, was offered in the Temple in Jerusalem, up until the day before an offering of wheat was brought to the Temple on Shavuot. The Counting of the Omer begins on the second day of Passover (the 16th of Nisan) for Rabbinic Jews, and after the weekly Shabbat during Passover for Karaite Jews, and ends the day before the holiday of Shavuot, the 'fiftieth day.'.

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Recommended Books

Counting the Omer: A Kabbalistic Meditation Guide
Counting the Omer
by Min Kantrowitz
Counting the Omer: A Daily Devotional Toward Shavuot
Counting the Omer
by William Mark Huey
Omer Teshuvah: 49 Poetic Meditations for Counting the Omer or Turning Toward a New Year
Omer Teshuvah
by Shifrah Tobacman

References

The Jewish Holidays: A Guide & Commentary
Rabbi Michael Strassfeld
"Counting of the Omer" in Wikipedia: The Free Encyclopedia
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