According to Jewish Law, it is forbidden to cut the hair of one's head during the period of the Counting of the Omer in remembrance of the 24,000 students of Rabbi Akiva who died as a result of a plague. The Talmud explains that they perished as divine punishment for not honoring one-another. The students stopped dying on the 33rd day of the Counting of the Omer, known as Lag B'Omer, and today many cease this semi-mourning at this time; others, like the Ari himself was accustomed, continue until the day before Shavuot, at the completion of 49 days.

The reason we don't cut hair during these days is because, as is known, there is a rectification [among the Sheva Tikunei Reisha, or 7 Rectifications of the head of Arich Anpin] in the [supernal] Skull [Gulgalta] called "Emer HaNaki" [literally "White Wool", associated with the hair which flows from the head of Arich Anpin] and [another] rectification called "Talei D'Bdulcha" [literally "Crystal Dew", associated with the brain].

As the Ari will elaborate, the word "emer" from "Emer HaNaki" is spelled the same as the word "omer", the measurement prescribed for the daily barley offering meant to be brought during this time period. Besides the Skull itself and the Cranial Membrane (Kruma D'Avira) the two above-mentioned of the seven supernal rectifications of the head of Arich Anpin, described by the Zohar1 are the two most supernal, through which all influx of divine wisdom descends to the lower worlds. The mitzvot associated with the period of the Counting of the Omer…actually help sweeten all of existence…

Emer HaNaki, understood as most as relating to the place of the ears of Arich Anpin, originates in the inner netzach and hod of Atik Yomin. Talei D'Bdulcha, the brain of Arich Anpin, i.e. its chochma, originates in the sefira of gevura of Atik Yomin; this is the supernal root of all judgmental forces in Adam Kadmon. Thus, we see that these two places are the source of most of the harsh forces to "descend" into the world we experience. By being conscious of the powers invested in us to mollify these powers - and put them into a useful context - via the mitzvot associated with the period of the Counting of the Omer, we actually help sweeten all of existence.

Regarding the requirement that the generation of the Exodus preserve a measure of manna as a remembrance to future generations, the Torah states:
Moses said to Aaron, "Take a jar and put a full omer of manna into it; place it before G‑d for safekeeping for your generations." (Ex. 16:33) A few verses later the amount required is further described:

This is the secret of the verse "The omer is a tenth of an ephah. (Ex. 16:36)

The Ari understands the "omer" in the above verse as relating to the "omer" of barley that we offer during the period between Pesach and Shavuot known as the Counting of the Omer.

…Thus two times the word "omer" is the numerical value of the word "keter" [literally "crown"], associated with the Skull [of Arich Anpin] and the Emer HaNaki, which is associated with the hairs [of the head of Arich Anpin], which are in [the sefira of] keter.

And here, "The omer is a tenth of an ephah" is the secret of the letters of the word "peah", which is associated with malchut, which is called "Elokim", which is the tenth of the [ten] levels [literally "middot"] - thus, "…a tenth of an ephah."

The words "ephah", the measurement, and "peah", literally "corner", as in the phrase "the four corners of the earth" (connoting this-worldliness), are spelled using the same letters.

Behold, Rabbi Akiva expounded upon these hairs, which is the secret of when he taught about the crowns of the letters. In the traditional Ashurite script of the Torah crowns resemble small hairs…

The Talmud (Menachot 29) describes Moses' astonishment after being given a prophetic vision of Rabbi Akiva expounding myriads of laws learned from the crowns found in the traditional Ashurite script of the Torah. These crowns resemble small hairs and are described in the Writings of the Ari as channels for the descent of supernal wisdom.

As a result, because his students perished, it is forbidden to cut hair during this period.

Rabbi Akiva's students were rooted in an aspect of his soul and were channels for his wisdom into this world. In commemoration for this loss, it is fitting that we, on a microcosmic level, act accordingly. Rabbi Tzvi Elimelech from Dinov, the Benei Yisachar, citing the Ari's teaching that Rabbi Akiva was rooted in the sefira of chochma, notes that the Hebrew word for "honor" ("kavod") - that which Rabbi Akiva's students were lacking in their treatment of their fellows - has the numerical value of 32, related to the 32 Paths of Wisdom (chochma). Issachar…was reincarnated in Rabbi Akiva…

Elsewhere, the Ari notes that the Hebrew word for "hair", "sayar", is rooted in the same letters as word for "barley", "seura", the actual commodity brought as the Omer offering.

The word "omer" has the numerical value of the letters shin-yud [310, as in the concept of 310] Worlds, "Yesh [spelled yud-shin] m'Ayin" [literally, "Existence from Nothingness"].

"Ayin" - nothingness - is related to the sefira of keter. Thus, the Ari hints that the entire supernal source of the strength of Rabbi Akiva is the same as that of the Omer period.

This is the [first] yud-shin of the name "Issachar" [the son of Jacob] - "…a strong-boned donkey…" (Gen. 49:14), who was reincarnated in Rabbi Akiva. This is the secret of [when Rabbi Akiva said] "I will bite you all, as would a donkey".

In his younger years, Rabbi Akiva, before returning to the fold, used to threaten the sages of his generation thus. In "Gate of Reincarnations", the Ari goes into great detail regarding the reincarnations of Rabbi Akiva, citing, in addition, that the majority of the members of the Sanhedrin, i.e. the greatest of the sages of each generation, were from the tribe of Issachar, whose strength lay in the sefira of chochma.

[Pri Etz Chaim, Shaar Sefirat HaOmer, Chapter 7; translated and edited by Baruch Emanuel Erdstein]

To find out more: Mystical Significance of Hair