For every individual to make a formal count of the days and weeks between the beginning of Passover and the holiday of Shavuos
50 days later, starting on the second day of Passover (the day the barley-offering is sacrificed in the Jerusalem Temple) and counting 49 days, ending the day before Shavuos
. Since Hebrew-calendar days begin at sunset, the counting ceremony is performed in the evening that begins each of the 49 days.
This count corresponds to the seven weeks the Jewish people spent in the desert from the time they left Egypt until they arrived at Mt. Sinai, and the modern act of counting helps bring about a spiritual refinement in a persons soul. Gentiles are not obligated in this commandment, but may perform it for extra reward from G-d though without reciting a blessing over the act (see rabbinical commandment 1).