The custom to eat dairy and honey on Shavu’ot is a custom that goes back over 600 years, to the time of the Rishonim. The custom originated in France and Germany, and spread from there to many Jewish communities. However, it is not universal. Many from Libya, Djerba, Bukhara, Iran, and Yemen do not follow it.
A number of reasons are given for this custom. Some say that it is because the Torah is compared to milk and honey (Devarim Rabba 7:3), as we read: “Honey and milk are under your tongue” (Shir Ha-shirim 4:11). According to another midrash, when the Jews stood at Sinai and said, “All the Lord has spoken we will faithfully do” (Shemot 24:7), God responded with the verse: “Honey and milk are under your tongue” (Tanḥuma Buber, Ki Tisa §9). The idea is that since the Jews agreed to accept the Torah unconditionally, they would find the Torah’s words as sweet as honey and milk. Accordingly, to remind us of the Torah’s sweetness and preciousness, there is a custom to eat dairy cakes that are tasty and sweet, as well as foods sweetened with honey (Orḥot Ḥayim; Pri Ḥadash).
Rav Kook presents a second explanation. Both milk and honey are foods which originate in something “impure” (not kosher). Honey is produced by bees, while milk is formed from blood. Because these foods involve the transformation of impure to pure and thus symbolize repairing the world, they have a special taste. Foods which symbolize transformation are appropriate to eat on the holiday celebrating the giving of the Torah, as the Torah has transformative power. It repairs what is wrong with the world, and “flavors” the evil inclination, transforming it into a positive force. Eretz Yisrael also possesses this power, which is why it is referred to as a land flowing with milk and honey.
A third suggested reason for the custom is that as a result of a Yom Tov menu including dairy foods, people have to prepare two challahs – one to be eaten with dairy and the second with meat. This hints at the offering of the shtei ha-leḥem on Shavu’ot (Rema 494:3). It has been further suggested that the custom imitates the behavior of our ancestors when the Torah was given. Right after they had received so many laws about preparing meat for consumption – including how to slaughter animals, how to check the slaughtering knife, how to salt the meat and more – they preferred to eat dairy. Dairy foods were easily prepared, requiring little work. To commemorate this, we too eat dairy on Shavu’ot (MB 494:12). However, since there is also a mitzva to be joyful on the festival, we eat meat then as well. We must be careful to separate between the two. By doing so, we demonstrate that we cherish the laws of the Torah.
Many have a custom to eat both milk and meat at the same meal. Some do this at the daytime meal, while many do it at night. They begin with dairy food. Afterward, they must brush their teeth or eat bread, an apple, or any other hard food, and then rinse out their mouths to wash away any milk residue. Then the tablecloth is changed, the table is reset, and meat is served. Obviously, the order cannot be switched, because if people begin with meat, they would need to wait six hours before having dairy.
Another variation practiced by many who stay up all night is to make kiddush after Shaḥarit in the morning, eat dairy cakes, and then go to sleep. After they wake up, they have a meat meal. Of utmost importance is the mitzva of simḥa; all these customs are meant to add joy to the holiday and honor to the Torah.