Dairy on Shavuot
There is a well-known custom to eat dairy foods on Shavuot at one or more of the holiday meals, or at least as a snack. This is especially true on the first day of Shavuot in the Diaspora. There are a number of explanations offered for this custom. It is explained that when the Jewish people received the Torah at Mt. Sinai (the first Shavuot in history) they immediately realized that their dishes weren’t kosher! This is because there had been no prohibition against mixing milk and meat before this time nor was there any requirement to perform shechita, ritual slaughter, in order to permit eating meat. So too, the procedure for preparing kosher meat is quite lengthy and it would have taken many hours for the Jewish people to prepare their first ever fully kosher meat meal. As such, they were left with little choice but to eat dairy meals that day. Eating dairy is also intended to recall that the Torah is compared to milk and honey. Milk also represents purity in Torah literature.
Another view is that eating both a meat and dairy meal on Shavuot serves to remind us of the shtei halechem, the two special breads that were offered in the Beit Hamikdash on Shavuot. By eating both a meat and dairy meal, one is required to prepare at least two loaves of bread – one for the meat meal and one for the dairy meal – symbolizing the shtei halechem. There is also a theory that the Jewish people may have never eaten dairy prior to the giving of the Torah out of concern that doing so would be a violation of "eiver min hachai" – the prohibition against eating the product of a [still] living animal. The Torah, once revealed, teaches us that it is indeed permitted to consume dairy products. It is also taught that milk can only be stored in simple earthenware vessels. If milk is stored in silver or golden vessels, it spoils very quickly. Likewise, eating dairy on Shavuot reminds us that one who is haughty or boastful will not succeed in his Torah studies – only one who is humble and simple like an earthenware vessel will succeed and grow in Torah.
Eating dairy on Shavuot is also intended to recall the angels that visited Avraham Avinu after he had his brit mila. Avraham served them both milk and meat foods upon their arrival. The Midrash teaches that when Moshe ascended Mount Sinai to receive the Torah, the angels protested and argued that the Torah belongs in Heaven and should not be given to mortal man. Moshe responded by reminding them that they did not keep the Torah when they visited Avraham Avinu, and ate milk and meat together! Moshe’s argument triumphed, of course, and the Torah was given to the Jewish people. To strengthen Moshe’s argument, we too, eat both milk and meat on Shavuot, being very careful to properly separate the two foods as required.
Additionally, the numerical value of milk (“chalav”) is forty, which recalls the number of days that Moshe spent on Mount Sinai in preparation for the receiving of the Torah. Incidentally, another name for Mount Sinai is Mount Gavnunim which sounds similar to "Gevina," - "cheese." It is also noted that the mitzva to bring the first fruits to the Beit Hamikdash on Shavuot is written in the Torah right after the prohibition against mixing milk and meat. This, too, reminds us that we must eat both types of food on Shavuot, being mindful of the required separation.
Some have the custom to eat a dairy meal on Shavuot night and a meat meal during the day. Others are of the opinion that the mitzva of "Simchat Yom Tov," often interpreted as a requirement to eat meat on Yom Tov, must be discharged at night. As such, many have the custom to eat a meat meal on Shavuot night and a dairy meal during the day. In some communities, especially Chassidic ones, the custom of eating dairy on Shavuot is observed with simply a dairy snack, such as at a Kiddush following the morning prayers.
There is an opinion that one may begin a meal with some light dairy foods, and then proceed to eat meat foods after cleaning one's hands and mouth as required. Most authorities, however, strongly discourage combining dairy and meat in the same meal at any time for any reason. So too, one is required to recite the Birkat Hamazon, or other bracha achrona, before eating food of the opposite type no matter how much time has passed. Furthermore, some authorities require one to wait one hour between dairy and meat foods.
 Rema, OC 494:3.
 Mishna Berura 494:12.
 Shir Hashirim 4:11.
 Magen Avraham 494:6.
Rema, OC 494:2. (Shemot 34:32; Vayikra 23:15-22).
 Machatzit Hashekel 494:7. See also Igrot Moshe, OC 1:160.
 Shemot 3:8.
 Moadim U'zmanim 8:319.
 Yalkut Shimoni, Ruth 597.
 Bereishit 18:8.
 Midrash Tehillim 8 (cf. Shabbat 88b -89a); Be’er Heitev 494; Minhag Yisrael Torah, OC 494:8.
 Tehillim 68(cf. Shabbat 89).
 Shemot 23:19.
 Minhag Yisrael Torah, OC 494:8. See also Revivot Ephraim 8:494:4,5,6.
 Rambam, Hilchot Yom Tov 6:17; Magen Avraham 529.
 Minhag Yisrael Torah, OC 494:8, Meishiv Halacha 1:25.
 Mishna Berura 494:16.
 Darkei Teshuva, YD 89:19.