Tosfos explain that Sefiras Ha’Omer is a rabbinical law, in memory of the destruction of the Beis HaMikdash. The reason this is so is that the pasuk says:
וּסְפַרְתֶּם לָכֶם מִמָּחֳרַת הַשַּׁבָּת מִיּוֹם הֲבִיאֲכֶם אֶת עֹמֶר הַתְּנוּפָה שֶׁבַע שַׁבָּתוֹת תְּמִימֹת תִּהְיֶינָה – And you shall count for yourselves, from the morrow of the rest day, from the day you bring the omer as a wave offering seven weeks; they shall be complete. (23:15)
The Rambam disagrees, and says it is a commandment directly from the Torah to count today. Looking at the above pasuk, it seems difficult to suggest this, as we don’t bring a Korban Omer – so how can we say there is a mitzva today to count the Sefira?
R’ Yaakov Minkus explains that there are two reasons for counting Sefira.
Tosfos say that the Sefira marks the beginning of the harvest cycle. Pesach marks the beginning, and is called ראש הקציר – the initiation of the harvest. It was marked through the Korban Omer, and allowed the consumption of new grain within the Beis HaMikdash. Shavuos marks the end, and is called חג הקציר – the celebration of the harvest. It was marked through the Shtei HaLechem, which allowed the consumption of new produce, outside the Beis HaMikdash, and everywhere.
The Sefira is the process of enabling produce. We return the first of the harvest, the beginning, to Hashem. The Korban Omer exists to show our fealty and identity with Hashem. The Sefira results from this.
The Rambam explains that the pasuk’s instruction is to count מִיּוֹם הֲבִיאֲכֶם אֶת עֹמֶר – from the day, that is to say, not from the Korban itself. So what is it we are counting from today then?
The Sefer HaChinuch teaches that counting Sefira takes us to Har Sinai. It is easy to think of Pesach as a standalone day where we celebrate our liberation – it is not so. Pesach was the sole means by which we could transition from slavery to Shavuos and receiving the Torah.
Shavuos is an annual occurrences. It occurs without the Korban Omer – this is how it can still be a mitzva to count Sefira. We count מִמָּחֳרַת הַשַּׁבָּת – from after Pesach, not the Korban itself.
When a person makes a bracha, they say אשר קדשנו במצותיו, וצונו. Women have the first part, the אשר קדשנו במצותיו, but not the second, וצונו. Har Sinai gave us the Torah and it’s 613 commandments in a general sense. The women accepted the Torah in a general sense too. Although women are exempt from many mitzvos, which is to say they don’t have to, it does not mean they cannot grow from their performance – this is קדשנו. A non-Jew who performs a mitzva does not have either part of the bracha, and cannot grow from the performance of a mitzva.
This is the difference between Pesach and Shavuos. Pesach is full of mitzvos; the Korban Pesach, circumcision, the Seder. In performing Hashem’s instructions, we became His people, subjugated to Him. This is all וצונו. However, Sinai is Torah. This gave mitzvos a קדשנו – which women are also subject to. Sefira connects the אשר קדשנו במצותיו to the וצונו.
But the Sefira is not “just” to connect Pesach to Shavuos, the mitzvos to the Torah; but also, the other way around, Shavuos to Pesach. The first of the Ten Commandments is אָנֹכִי ה’ אֱלֹהֶיךָ אֲשֶׁר הוֹצֵאתִיךָ מֵאֶרֶץ מִצְרַיִם מִבֵּית עֲבָדִים – (to know that) “I am the Lord, your God, Who took you out of the land of Egypt, out of the house of bondage”. How can the first (or any) commandment be faith?
The first command specifies that Hashem took us out of Egypt. Egpyt and faith are inextricably linked. We were not liberated from Egypt at all – we were transferred. In the same way we recognised that Hashem had taken us out of Egypt, that same Hashem was giving us the Torah, based on a belief in Him. Sinai’s eternity is based on having been taken out of Egypt, and in this way Sefira links Shavuos and Pesach, both backwards and forwards.