הלכה על מנחות 131:13

That is that He commanded us to count forty-nine days from the harvesting of the omer. And that is His, may He be exalted, saying, "you shall count for yourselves seven weeks from the morrow of the Shabbat, etc." (Leviticus 23:15). And you should know that just like the court is obligated to count the years of the Jubilee, year by year [and] cycle by cycle - as we explained in what came previously (Sefer Hamitzvot, Positive Commandments 140) - so too is each and every one of us obligated to count the days of the omer, day by day and week by week. And that is His having said, "you must count fifty days" (Leviticus 23:16), and "Seven weeks shall you count for yourself" (Deuteronomy 16:8). And just like the commandment of counting the years and the sabbatical cycles is one commandment - as we explained - so too is the counting of the omer, one commandment. And do not be fooled by their saying (Menachot 66a), "It is a commandment to count days and it is a commandment to count weeks," and think that they are two commandments. For it is a commandment to do each and every part of commandments that have have different parts. However had they said, "From where [do we know that] the days are a commandment; and from where [do we know that] the weeks are a commandment" - it would have been two commandments. And this will not be lost on the one that will not be fooled by the words. Since were you to say, "It is an obligation to do such and such," it surely does not necessarily make that action a separate commandment. And the clear proof for this is our counting every night that it is such and such weeks and such and such days. And were it to be a separate commandment, we would only arrange its content on the night of Shavuot; and we would say two blessings - on the counting of the omer; and on the counting of the weeks of the omer. But the things are not like this. Rather the commandment is counting the omer daily and weekly [together], as they ordained. And women are not obligated in this commandment. (See Parashat Emor; Mishneh Torah, Daily Offerings and Additional Offerings 10.)

From the laws of the commandment is that which they, may their memory be blessed, said (Menachot 66a) that it is a commandment to tally them from the evening so that they be complete. As the verse stated, "complete shall they be" - and they, may their memory be blessed, said, "From when are they complete? From when he begins from the evening." And nonetheless, the commentators (Tosafot in the name of Behag on Menachot, s.v. zecher) explained that if he forgot and did not tally from the evening, he [may] tally on the morrow the whole day. And some say there that one who forgot and did not tally one day may not tally again that year, since they are all one commandment; and since he forgot one day from them, the entire count is negated for him. And our teacher in our generation did not concede to this reasoning. Rather, one who forgot a day should say, "Yesterday was such," without a blessing; and tally the other [days] with all of Israel. And it is a choice [fulfillment] of the commandment to tally [while] standing (Mishneh Torah, Laws of Daily Offerings and Additional Offerings 7:23). And he recites the blessing, "who has sanctified us, etc." And one who has tallied without a blessing has fulfilled [the commandment], and [hence] he is not permitted to return and tally with a blessing (Mishneh Torah, Laws of Daily Offerings and Additional Offerings 7:25). And the rest of its details are in Tractate Menachot (see Tur, Orach Chaim 479).