When the Jewish People left Egypt they counted forty-nine days, each representing the rectification of one spiritual level in preparation for receiving the Torah. Today as well we must look to purify and elevate ourselves in preparation for receiving the Torah. In fact, the Zohar explains the great importance of each individual counting the Omer during these days as a preparation for learning Torah. It is also a custom throughout the Jewish People to study “Pirkei Avot”, Ethics of the Fathers, during these weeks in order to help refine one’s character traits before receiving the Torah.
The majority of halachic authorities maintain that today the command to count the Omer is Rabbinical, instituted as a remembrance for the Beit Hamikdash. From the second night of Pesach (the first night of Chol HaMo’ed in Israel, or the second night of Yom Tov outside of Israel) we begin counting the Omer at the conclusion of the evening prayer. (Shulchan Aruch 489:1). According to the Mishneh Berurah the counting is done before Aleinu, while Sefardim count immediately after the completion of the Ma’ariv evening service.
Women are exempt from counting because this is a positive mitzvah that is “bound by time”. On this point the Magen Avraham rules that although women are technically exempt, and therefore do not need to count, they have nevertheless accepted upon themselves to count. However, the Mishneh Berurah writes that women in his city did not have the custom of counting at all. Today, in some communities women count, while in others they do not.
The ideal time for counting the Omer is at the start of the evening, but if one forgets he can still count all night, with a blessing. If one forgets to count in the evening he can count the following morning, which is the same calendar day in Judaism, but without a blessing. And then he can continue to count on the following evening with a blessing. The general rule is that as long as the person has counted every day, he continues to count with a blessing. But if he misses one day completely, he can no longer make a blessing. However, it is important to note that he still continues counting without a blessing.
Rabbi Ovadia Yosef explains, based on the Chida, a case in which although one has not missed a day of counting, he nevertheless must stop counting with a blessing. If the person becomes Bar Mitzvah during the Omer, although he has counted every day, he has done so only because of “chinuch”, to teach and accustom him to correctly perform the mitzvot when he is an adult and obligated by the Torah. Now that he is Bar Mitzvah he has a new obligation to count, and since he cannot count all of the forty-nine days as an adult he can no longer count with a blessing. The Maharam Shick and and Tzitz Eliezer disagree, and rule that the blessing can still be made when he counts in this scenario.