(First and last paragraphs added by KabbalaOnline)

Many explanations are given of our ancient custom to eat dairy foods on the festival of Shavuot. One reason given is that upon receiving the Torah the Jewish People realized that their pots and slaughtering knives were not kosher (Geulat Yisrael), and so the people had to eat foods not requiring slaughtering (i.e. milk products). Another reason is that the numerical value of the Hebrew word for milk, "chalav", is the same as the number of days Moses spent alone on Mount Sinai with G‑d to receive the tablets (Rabbi Shimshon of Ostropole). However, reasons based on Kabbala are not so well known. Here is one:

Pesach…is associated with the blood of birth….

Pesach, and the Exodus process in general, is associated with the blood of birth, as it is written, "I passed over you and saw you wallowing in your blood; and I said to you, 'In your bloods you shall live!'" (Ezekiel 16:6).

Shavuot, on the other hand, is associated with Torah, which is like milk, as it is written, "Honey and milk [i.e. the secrets of the Torah] are [guarded] beneath your tongue". (Songs 4:11)

The meaning of the symbolism is that the Children of Israel were born on Pesach. Birth is clearly associated with blood. On the other hand, according to our sages, nursing involves the transformation of blood into milk (Bechorot 6b). Just as the birth process is amazing and miraculous, so also is this transformation of blood (which was the sole medium of nourishment for the baby while in utero) into milk. This is why, generally speaking, as long as a mother nurses her child, she cannot become pregnant, for the nursing (if it constitutes the exclusive source of nourishment for the infant) prevents ovulation.

The mother's birth blood of Pesach has been transformed into the mother's milk of Shavuot….

Thus following this symbolism, we see that during the intervening 49 days of the Omer, the mother's birth blood of Pesach (which is red) has been transformed into the mother's milk of Shavuot (which is white).

This also corresponds to the tradition that is preserved in Kabbala that Zeir Anpin was in a state of gestation during the Egyptian exile. On Pesach, the baby emerged, so to speak, from the birth canal. Then the symbolism switches to a young girl who is about to marry. She must wait 7 days after her period before she can have relations with her husband. So also on the cosmic scale, the nation of Israel must wait 7 x 7 (49) days before she can enter under the wedding canopy of Sinai, to "marry" the Holy One blessed-be-He.

This fits with the Talmudic teaching in which King David praises and thanks G‑d that human breasts are not below (paralleling the birth canal) like an animal's, but above, on the chest (paralleling the heart). (Berachot 10a) So too here, we have risen up from the birth canal to the breasts.

May our eating dairy foods on Shavuot correspond to our receiving the sweet, nourishing words of Torah in a manner that fulfills the verse, "Honey and milk are under your tongue". (Songs 4:11)