What are the reasons mentioned behind eating dairy products on Shavuos?
- Commemoration of Shtei Halechem: Dairy products are eaten in commemoration of the Shtei Halechem offering, which consisted of two loaves [i.e. two meals], which were brought on Shavuos. We thus eat two meals with bread, one dairy and one meat, to emulate the two loaves.
- Like a pure woman: In preparation for Matan Torah we count seven weeks of Sefiras Haomer. This count is similar to the seven clean-day count of a Nida, in preparation for her immersion. Now, it is known that blood serves as the source of the milk during nursing, and represents the purification the Nida blood, to life giving nutrients found in milk. For this reason, on Shavuos, we eat dairy to emphasize that the seven weeks of purification have effectively come to a close, and we are now purified of all evil.
- Torah is allegorized to milk: The Torah is allegorized to milk, as states the verse “Devash Uchalav Tachas Leshonecha”, and hence we eat dairy on Shavuos, which is the day of the giving of the Torah.
- Commemoration of after Matan Torah meal: Some suggest that the Jews did not keep Kosher until Matan Torah, and hence when they arrived home after Matan Torah they had no meat to eat, as it was all Treif. They likewise could not cook anything, even Pareve, being that all their vessels were Treif. They had no option but to eat cold dair, which was readily available. Some suggest that bewfore Matan Torah we kept Kosher, but we were allowed to eat non-slaughtered meat. Once the Torah was given and we became obligated in eating slaughtered meat, all of our meat became Treif. We could not slaughter new meat after Matan Torah because the Torah was given on Shabbos, and it is forbidden to slaughter on Shabbos, and hence had to resort to eating dairy. The Rebbe suggest that in truth we did keep Kosher even before Matan Torah. However, our Shechita which we performed befopre Matan Torah became invalid when we became Jews after Matan Torah, and thus all of our meat and meat vessels became not Kosher. For this reason, only dairy products were available to be eaten.
- Commemoration of the reason we merited receiving the Torah: The Midrash famously states that the supernal angels attempted to litigate against the giving of the Torah to the Jewish people for various reasons. The final rebuttal, or comeback, which closed all litigation and allowed the Torah to be given involved the Mitzvah of not eating meat and milk together. The argument was as follows: After the angels told Hashem that they desired to keep the Torah for themselves, Hashem answered the angels that it states in the Torah “Thou shall not eat a kid in its mother’s milk.” Now, you angels surely remember the meal you ate in the home of Avraham Avinu? You ate meat and milk together during that meal as the verse states “Vayikach Chema Vechalav,” so how can you now ask to receive the Torah? This, states the Midrash, was the final comeback which refuted any claims from the angels, and allowed the Torah to be given. An even deeper approach is stated in the name of the Alter Rebbe: The law is that while one may not eat dairy after meat, one may eat meat after dairy. The above Midrash is hence puzzling, as the verse explicitly states that it offered first dairy and then meat to the guests, and they therefore performed no transgression. The explanation is as follows: The reason for the prohibition against eating meat and milk together is because milk is from Chesed and meat is from Gevura, and their combination can be catastrophic. However, this only applies if the Gevura overpowers the Chesed, while if the Chesed overpowers the Gevura then it is actually a positive matter. Now, we have a general rule of Tatah Gavar, the bottom overrules, and hence if one first eats dairy, he may eat meat afterwards, as the bottom which is dairy/Chesed, overrules the meat/Gevurah. However, if one eats meat first, then the Gevurah overpowers the Chesed. This system however only applies in this world, in which we hold that the lower realms are of greater importance than the higher realms, and hence the lower item overpowers. However, in the Heavens, they believe that the higher realm is greater than the lower realms, and therefore the rule of Tatah Gavar does not apply. Accordingly, Hashem told the angels that if the Torah is given to them, and thus the higher realms prevail, it would end up that they ate meat and milk together, hence transgressing the Torah. Furthermore, by the mere fact that by Avraham they agreed to eat first dairy and then meat shows that they too agree that the lower realms overpower, and hence they have no claim to receive the Torah. [Accordingly, we can explain the custom of eating a dairy meal on Shavuos, and then eating a meat meal, as this commemorates the victorious rebuttal which gave us the Torah to begin with. It also emphasizes that the purpose of the Torah is for the lower realms.]
- White is Chesed: The color white is a color of Chesed, and the giving of the Torah was a great kindness. It is for this reason that we eat dairy on Shavuos.
- Har Sinai was a “cheese cake”: The Midrash says that Har Sinai had seven names, one which is Har Gavnunim” which means a mountain of cheese. This means to say that it was as clean as a white cheese cake. We thus eat dairy on Shavuos to commemorate this matter.
- Celebrating the allowance to eat dairy: Prior to Matan Torah, dairy products were forbidden to be eaten due to Eiver Min Hachaiy. Therefore, to commemorate the allowance of eating dairy products which began after Matan Torah, we eat dairy on Shavuos.
Must one wash for bread by each of the two meals, dairy and meat, or does it suffice to wash for the meat meal and have Mezonos for the dairy meal?
Some are accustomed to wash on bread for both the meat and dairy meal. Nevertheless, the widespread custom is not to eat bread for the Milk meal and rather to eat Mezonos, and then wash on bread by the meat meal.
 Rama 494/3 “In certain places it is customary to eat dairy on the first day of Shavuos and in my opinion the reason is because it is simailr to the two foods we take on Pesach, on the night of the Seder, to commemorate the Karban Pesach and Karban Chagiga. So too, on Shavuos we eat dairy and afterwards eat meat, [as one may not use the same bread for both a meat and dairy meal-Machatzis Hashekel 494/7]. Hence, the two meals of meat and dairy force us to bring two loaves of bread [one for each meal] to the table, which is in place of the altar. This commemorates the two breads which were offered on Yom Habikkurim.”
 M”A 494/6 in name of Zohar; Beir Haiytiv 494/8; Aruch Hashulchan ibid
 Kol Bo 52; Chok Yaakov 494/9; Aruch Hashulchan 494/5
 M”B 494/12 in name of a Gadol; Sefer Geulas Yisrael ; Imrei Pinchas [Koretz]
 Imrei Pinchas [Koretz]
 Shaareiy Halacha Uminhag 2/194
 Midrash Raba Yisro 28
 Midrash Tehilim 8; Daas Zekeinim on Vayeira ibid
 Vayeira 18/8
 Brought in Sefer Pardes Haaretz [Horawitz] Vol. 3 p. 548 in footnote
 See Michaber Y.D. Chapter 89
 Bnei Yissachar Shavuos, brought in Taamei Haminhagim 622
 Taamei Haminhagim 624
 Taamei Haminhagim 624
 See Shach 87/9
 This is in accordance to the reason of the Rama 494/3, mentioned above, that dairy products are eaten in commemoration of the two loaves of bread that were offered on Shavuos. Accoridng to this reason, the main aspect of the custom of eating dairy is to eat two meals of bread, and thus commemorate the two breads. [So is implied from Rama 494/3 and Magen Avraham 494/8] This is also mention in Admur 494/16 when he discusses the custom to knead bread with milk. Tammeiy Haminhagim likewise writes that one eats two meals, washing for both the meat and milk meal. See Piskeiy Teshuvos 494/12
 Darkei Teshuvah 89/19 “as he received from his ancestors”; Darkei Chaim Veshalom 644; Piskeiy Teshuvos 494/12; Hiskashrus 931; In the Chabad Tikkun Leil Shavuos it writes that one does not wash for the meal, but rather eats Mezonos after Kiddush and then says a Bracha Achrona and washes for the meat meal. Similarly in Otzer it says that the Rebbe only ate mezonos for the milk meal. Perhaps this can also be inferred from Admur which writes that “it’s a custom to eat milk products” and does not write that “it’s a custom to eat two meals”.