Before counting the Omer, one recites the following blessing: “Blessed are you, O Lord, our God, King of the Universe, Who has sanctified us with His commandments and commanded us regarding the counting of the Omer.” Both the blessing and the counting are said, le’chatchilah, in a standing position. If one said them while sitting, he has nonetheless fulfilled his obligation (Sh.A. 489:1).
There are two components to the count – counting the days and counting the weeks, as it says, You shall count for yourselves from the morrow of the “Sabbath” – from the day you bring the Omer of waving – seven weeks; they shall be complete. Until the morrow of the seventh week, you shall count fifty days (VaYikra 23:15-16).
Therefore, one must mention the tally of days and weeks when counting the Omer (Menachot 61a). For example, on the seventh day, one says, “Today is seven days, which are one week, [of the Omer],” and on the fourteenth day, one says, “Today is fourteen days, which are two weeks.” We mention the number of days and weeks even in the middle of a week. For example, on the tenth day, we say, “Today is ten days, which are one week and three days.”
There are several versions of the text of the Omer count. Some say, laOmer (“of the Omer”), while others say baOmer (“in the Omer”). Some say, “Today is fourteen days of/in the Omer, which are two weeks,” and some say, “Today is fourteen days, which are two weeks, of/in the Omer.” One fulfills his obligation no matter which version he uses. The custom is to add the LeSheim Yichud paragraph before counting, as well as various other prayers afterwards, but one is not obligated to do so. The main components are the actual counting and the blessing preceding it.
The number seven alludes to a complete phenomenon, for the world was created in seven days. Indeed, every physical entity has six sides – four sides, a top, and a bottom – plus a seventh aspect, its inner core. Man, as well, has seven sides, which is why it takes seven days to go from a state of impurity to one of purity. For seven days, a person prepares all of his aspects to make this transformation.
The same is true of purifying oneself for sacred endeavors in this world, like eating terumah and sacrificial foods, as well as a woman’s purification process for her husband. However, in order to absorb God’s Torah, whose lofty stature belongs to the supernal worlds, we need to count much deeper – seven weeks instead of seven days. In this count, each one of the seven numbers appears in all of its seven facets. This way, our purification in advance of the giving of the Torah is complete. Every aspect of our character undergoes refinement and expresses its yearning and anticipation for the receiving of the Torah.
If, at the end of a week, one forgets to count the days, he does not discharge his obligation, even be’di’avad, and he must re-count with a blessing. And if he fails to rectify his mistake that day, he must count without a blessing from then on. If, at the end of a week, one forgets to count the weeks, some say he fulfills his obligation, be’di’avad, while others say he does not. In the middle of a week, however, like the eighth day, if one says merely, “Today is eight days,” he fulfills his obligation, be’di’avad. And if he says, “Today is one week and one day,” some maintain that he fulfills his obligation. In all of these scenarios, one should re-count properly without a blessing, but if he failed to do so that day, he counts the next day with a blessing (based on M.B. 489:7, Sha’ar HaTziyun 9, 19, and halachah 2.8, below).