The Counting the Omer reference article from the English Wikipedia on 24-Apr-2004
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Counting the Omer

The Counting of the Omer is a 50-day count begun on the second day ofPassover and culminating in the holiday of Shavuot. The barley harvest in Israel begins during Passover; the omer is a Biblical measure of volume of grain. An omer offering was brought to the Temple on the second day of Passover and was the signal for the allowance of "chadash" grains.

Many Jews adopt certain customs related to mourning during this period, until the 33rd day of the Omer, called Lag b'Omer.