The Omer refers to the 49-day period between the second night of Passover (Pesach) and the holiday of Shavuot. This period marks the beginning of the barley harvest when, in ancient times, Jews would bring the first sheaves to the Temple as a means of thanking God for the harvest. The word Omer literally means “sheaf” and refers to these early offerings.
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. Wetherefore begin the counting of the Omer on the second night of Pesach, and end the night before Shavuot.
The Omer is a verbal counting that we count each evening after sundown. The counting of the Omer is generally added to the end of Ma’ariv (the evening service). One must stand when counting the Omer and one begins counting with a blessing. Each day is counted, and after six days, the weeks are counted as well. For example, on the first day one will say “today is the first day of the Omer”, and on the 8th day, one will say “today is one week and one day of the Omer”.
The Omer period is also a time of partial mourning in memory of a plague that killed dozens of Jewish scholars during the lifetime of Rabbi Akiva. As such, Jewish tradition prohibits weddings, parties, live music, dancing, and haircuts.
The 33rd day of the Omer (the eighteenth of Iyar) is a minor holiday commemorating a break in the plague. Today it is known as Lag B’Omer - the word “Lag” is not really a word at all; it is the number 33 in Hebrew as if you were to call the Fourth of July “IV July” (IV being 4 in Roman numerals). The mourning practices of the omer period are lifted on that date.
Lag B’Omer is also the anniversary of the death of the great sage Rabbi Shimon Bar Yochai. Every year thousands of Jews ascend Mount Meron outside the city of Sefat to celebrate his life.
Which is one week and four days of the Omer