Starting from the night of the Omer harvest, there is a mitzvah to count forty-nine days, which are seven weeks. The Omer is harvested on the sixteenth of Nissan, which coincides with the night after the first day of Pesach. That night, [our ancestors] would go out [to the fields], cut down stalks of barley, bring them to the Temple courtyard, thresh them, winnow them, separate out the chaff, toast the grains, grind them well, produce a tenth of an eifah of flour, sift it in thirteen sifters, mix it with a log [measure] of oil, and place upon it a kometz (around ¾ of a handful) of levonah (frankincense). The next day, [part of the mixture] would be offered on the altar. First, a kohen (priest) would wave it; and then he would separate a kometz [approximately ¾ of a handful, from the mixture] and burn it on the altar. After the kometz was burnt up, everyone was permitted to eat from the new grains.
It is important to know that the holiday of Shavu’ot does not have a calendar date like the other holidays do. For example, Pesach begins on the 15th of Nissan and Sukkot on the 15th of Tishrei. The date on which Shavu’ot falls, however, is determined by the Omer count. The holiday arrives after the seven-week count is completed, which is why it is called Shavu’ot – the Festival of Weeks. This is the meaning of the verse: You shall count for yourself seven weeks; from when the sickle begins [to cut] the standing crop shall you begin to count seven weeks. Then you shall observe the Festival of Weeks for the Lord your God (Devarim 16:9-10). It also 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, and you shall offer a new meal offering to the Lord (VaYikra 23:15-16).
This mitzvah is not incumbent upon the [Supreme] Rabbinic Court alone; rather, every Jew is commanded to count forty-nine days. And everyone must verbalize the sefirah (count) himself. In general, we have a rule when it comes to mitzvot that involve speech: “One who hears is as one who responds.” Therefore, for example, one can fulfill the mitzvah of remembering Amalek by hearing the reader [read Parashah Zachor]. Similarly, one can fulfill his obligation to recite a blessing over the counting of the Omer by hearing the leader’s blessing. With regard to the counting itself, however, several poskim hold that everyone must enunciate the count himself, as it says, You shall count for yourselves (Levush, Chok Ya’akov). True, others hold that the law of sefirah is identical to that of other speech-related mitzvot and one may therefore discharge his obligation by hearing someone else’s count (Pri Chadash, Birkei Yosef). Nevertheless, ideally, we try to satisfy all opinions; therefore, everyone must count for himself (see M.B. 489:5 and B.H. s.v. u’mitzvah).
The foundation of this mitzvah is rooted in our national inception. Chazal explain that the Children of Israel descended to the forty-ninth level of impurity during their bondage in Egypt. This made them unworthy of receiving the Torah and necessitated a purification process. Therefore, HaKadosh Baruch Hu waited seven weeks to enable them to purify themselves from the defilement of Egypt and reach a state in which they could receive the Torah (based on Zohar, Emor 97). The sefirah also expresses our anticipation for the giving of the Torah. The Midrash relates that when Moshe told the Jews that after leaving Egypt they would serve God on Mount Sinai and receive the Torah, they asked, “When will this service take place?” Moshe answered, “Fifty days later.” Then, due to their great love [for HaShem], they counted every day and said, “Behold, one day has passed; two days have passed,” and so on. On account of their love and anticipation for the Torah, it seemed to them as a long time (Shibolei HaLeket 236).
Thus, Sefirat HaOmer expresses our yearning for that great day, the day on which HaShem gave us the Torah, while we simultaneously undergo a process of purification in all the forty-nine levels of which man is comprised. The purer and “cleaner” one is, the more he will be able to absorb the Torah’s light. In this way, we prepare ourselves every year for the receiving of the Torah by way of the Omer count (see the end of halachah 3, below).