And the 161st mitzvah is that we are commanded to count [the days beginning with the offering of] the Omer.1

The source of this commandment is G‑d's statement,2 "After the [Passover] holiday you shall then count [seven complete weeks]."

You should keep in mind that just as the court [beis din] is required to count the years of the Jubilee cycle — each year and each Shemitah cycle, as we explained above3 — so too each one of us is required to count the days of the Omer, each day and each week.

[We know that one must count the days] from the verse4 "You shall then count [until5] 50 days." [We know that one must count the weeks] from the verse6 "Count seven weeks for yourself." Just as counting the years and Shemitah cycles is one single commandment, as we explained,7 so too counting the Omer is one single commandment [not two commandments, one for the days and another for the weeks]. All those who preceded me also count it as a single commandment, and did so correctly.8

Do not be misled to consider [the counting of days and weeks as] two commandments because of the statement of our Sages,9 "It is a mitzvah to count the days, and it is a mitzvah to count the weeks."10 [They use the expression, "It is a mitzvah"] because for any mitzvah that has many parts, it is a "mitzvah" [i.e. we are commanded] to do each part. If the Sages would have said, however, "Counting the days is a mitzvah, and counting the weeks is a mitzvah," they would be considered two separate commandments.11 This is clear to anyone who thinks carefully about the wording; because when it is said that there is an "obligation" to do a certain act, that expression doesn't necessarily indicate that it is a separate commandment.

The clear proof of this [i.e. that counting the days and weeks are not separate commandments] is that we count the weeks every single night by saying, "It is this number of weeks and this number of days." If [counting] the weeks would be a separate commandment, [the Sages] would have established them to be counted only on those nights which [complete] the weeks. They also would have established two blessings: "[Blessed are You G‑d, King of the universe,] Who has sanctified us with His commandments and commanded us to count the days of the Omer," and, "to count the weeks of the Omer." This is not the case; rather the mitzvah is to count the days and weeks of the Omer as was commanded.

Women are not obligated in this commandment.12