According to the Torah (Lev. 23:15), we are obligated to count the days from Passover to Shavuot – a period known as the “Counting of the Omer.” On the second day of Passover, in the days of the Temple, an omer (a unit of measure) of barley was cut down and brought to the Temple as an offering. This grain offering was referred to as the “Omer.”
Every night, from the second night of Passover to the night before Shavuot, we recite a blessing and state the count of the omer in both weeks and days. The counting is intended to remind us of the link between Passover, which commemorates the Exodus, and Shavuot, which commemorates the giving of the Torah. It reminds us that the redemption from slavery was not complete until we received the Torah.
This period is a time of partial mourning – during which weddings, parties, and dinners with dancing are not conducted and haircuts are forbidden – in memory of a plague that afflicted the students of Rabbi Akiva. The Talmud tells us that Rabbi Akiva had 24,000 students who tragically died during the Omer period, because they did not treat each other with sufficient respect. The 33rd day of the Omer (the eighteenth of the Hebrew month of Iyar) is a minor holiday commemorating a break in the plague. The holiday is known as Lag B’Omer. The mourning practices of the Omer period are lifted on that date.
The Orthodox Union (www.OU.org), Aish (www.aish.org) and other organizations have charts online that provide the transliterated Hebrew and English text of the counting day-by-day.
Source: My Jewish Learning