Lag B’Omer (the 33rd day of the Omer) is a special day, when the Light starts to enter Malchut from the Upper Sefirot. The Light comes to us from Infinity, passing through the World of Adam Kadmon, then the World of Atzilut, then through Partzufim Atik, Arich Anpin, Aba ve Ima, and Zeir Anpin, until it reaches the souls.
Zeir Anpin contains seven Sefirot, each of which contains seven Sefirot. Altogether, 7×7=49. Malchut is the 50th Sefira. That is how we count the days of the Omer, the entrance of the Light into Zeir Anpin, and on the 50th day, into Malchut. That is when our souls, which are included in Malchut, begin to receive the Light.
The unification (Zivug) of Zeir Anpin with Malchut takes place on the 50th day, but the Light enters Zeir Anpin completely on the 33rd day, when it enters Sefira Hod de Hod. Thus, there is Hesed, Gevura, Tifferet, and Netzah, with seven Sefirot inside each one of them. This adds up to 28, and there are five more Sefirot in Hod (Hesed, Gevura, Tifferet, Netzah, and Hod), which equals 33 (Lamed Gimel – Lag).
On this day we celebrate the entrance of the Light into Malchut of Zeir Anpin, meaning the Upper Light enters the system beneath which it already relates to the souls. If it reaches this place, then there’s no doubt that we will receive this Light. We begin to feel it and to prepare for its revelation.
The time from the 33rd day until the 50th day is already preparation. The Light continues to descend lower down and adapts itself for entering the souls. A person who is already in the spiritual world feels this preparation, which takes place before the reception of this special Upper Light, called Zohar.
That is why we celebrate this day. Rabbi Shimon left this world because he completed his mission. He no longer had anything to accomplish in this world because he passed the Upper Light through all the Sefirot down to Malchut de Zeir Anpin. With this, he completed his work.
Together with his students, he conducted the entire Light of Infinity through himself like through a special filter, preparing it to be received by our souls. This is the main work they accomplished. Now, when we look inside this book, even without understanding the words, with our desire we unite with the system they prepared for us, and through it we receive the Light.