The Torah states (Vayikra 21, 15): “And you shall count for yourselves, from the day following the Shabbat, from the day the waved Omer offering is brought, seven complete weeks shall they be.” Our Sages (Menachot 65b) have a tradition that the “day following the Shabbat” refers to the day following the first day of Pesach which is a holiday. (This is what is meant by the words, “the day following the Shabbat,” i.e. the day following the first day of Pesach which is a holiday, also known as “Shabbaton.” Therefore, on the night following the first day of Pesach following Arvit, we immediately begin counting the Omer.) It is a Torah commandment to count the Omer beginning from the Sixteenth of Nissan until the end of seven weeks, which is a period of forty-nine days.

Counting the Omer-A Torah or Rabbinic Commandment
Nevertheless, since the Torah also states (Devarim 16, 9), “You shall count for yourselves seven weeks, from when the sickle begins to strike the standing stalks shall you begin to count these seven weeks,” which means from the time the Omer offering was harvested and unfortunately nowadays when the Bet Hamikdash no longer stands, we have neither the harvesting of or bringing of the Omer offering. Thus, this Mitzvah of counting the Omer is only rabbinic in commemoration of the services performed in the Bet Hamikdash. Therefore, in the “Leshem Yichud” text customarily recited before counting the Omer, one should omit the phrase, “As the Torah states, ‘And you shall count for yourselves’” etc. for the Mitzvah of counting the Omer is no longer a Torah commandment. (Although according to the opinion of the Rambam and the Ra’avaya there is no correlation between the Mitzvah of counting the Omer and the harvesting the Omer and according to them the Mitzvah of counting the Omer is a Torah commandment even nowadays, nonetheless, we do not rule this way and the Halacha in this matter follows Maran Ha’Shulchan Aruch, whose rulings we have accepted, who rules that counting the Omer is only a rabbinic commandment nowadays, for this is indeed the opinion of Rav Hai Gaon, Tosafot,  Rosh,  Itur,  Rashba, Ran, and others).

One Who Forgets to Count One Day
The Mitzvah of counting the Omer is a Mitzvah during every single day of the counting period, and for this reason, we recite a blessing on it before counting every single day.

However, according to the opinion of the Ba’al Halachot Gedolot, if one has forgotten to count the Omer on one day during the counting period he can no longer continue to count the Omer since it is not possible to count by skipping (for if one counts one, two, four, he has counted incorrectly; thus, if one missed counting one day he can no longer rectify this and what he counts from now on is not considered counting at all). Halachically speaking, we hold that even if one has forgotten to count one day of the Omer, he may, in fact, continue to count the rest of the days for every day is a separate Mitzvah regardless of the other days. Nevertheless, since we always follow the great rule of “when in doubt, do not bless,” regarding the blessing we are concerned about the opinion of the Ba’al Halachot Gedolot. Therefore, if one forgets to count one day of the Omer, one should continue to count the rest of the days as usual; however, from now on he should not recite the blessing before counting.

A Child who Turns Thirteen during the Omer
Maran Rabbeinu Ovadia Yosef zt”l  discusses (in his Responsa Yabia Omer, Volume 3, Orach Chaim, Chapter 27) the Halacha regarding a child who turns thirteen years old during the Omer period. Since the days he has counted until this point were counted when he was still a child and not halachicaly obligated to perform the Mitzvot like an adult, his status is similar to one who has not counted the Omer until now and thus does not continue to count with a blessing on subsequent nights. He speaks lengthily and brings a great many sources to support his view. The luminaries of the previous generation debated this matter at length. Nevertheless, halachicaly speaking, a child who turns thirteen years old during the Omer should continue to count on a subsequent night without reciting a blessing.

The Time for the Counting and the Laws of Women and Counting the Omer
The appropriate time for counting the Omer is at night; however, if one forgets to count at night, one may count throughout the day without reciting a blessing before counting, in which case one may continue counting on all subsequent nights while reciting a blessing.

Women who count the Omer should not recite a blessing before counting. According to our custom though, women do not count the Omer at all. The reason for this is discussed by the Mekubalim.