There is a widespread minhag on Shavuos to eat dairy foods. (Remo on Orach Chaim 494:3). The Mishnah Berurah (ibid. s.k. 12) proffers the famous explanation for this custom: that Bnei Yisroel, upon receiving the Torah on Shavuos, were unable to eat meat right after the Torah was given. There was no time to prepare and check shechitah knives, remove blood and cheilev (non-kosher fats) from meat, and kasher utensils needed to cook and prepare hot meat. Thus, it was necessary on that first Shavuos to consume cold milchige foods. We therefore commemorate this event by also partaking of dairy dishes on Shavuos.

The Remo himself offers another rationale for eating dairy food on Shavuos: The korbon (sacrifice) of Sh’tei Ha-Lechem, the “Two Breads”, is commanded to be brought on Shavuos, therefore, we eat both dairy and meat foods, as this will require us to have two different breads (because we cannot eat the same bread with milchige and fleishige foods); the two breads necessitated by serving dairy and meat dishes, served on the table, which symbolizes the Mizbayach (Altar), commemorate the Korbon Sh’tei Ha-Lechem. (MB ibid s.k. 14)

There are some other, less-known explanations as to why we eat dairy foods on Shavuos:

Moshe Rabbeinu was taken out of the Nile on Shavuos and was thereafter brought to be nursed, and he refused to drink milk from non-Jewish women; the gematria of “cholov” (milk) is 40, representing the forty days that Moshe was on Har Sinai; one of the names of Har Sinai is “Gavnunim”, similar to the word “gevinah” - cheese. The Chok Yaakov (OC ibid. s.k. 9) quotes the Kol Bo (s. 52) that the minhag is to eat both honey and milk on Shavuos, as the Torah is compared to honey and milk (Shir Ha-Shirim 4:11).

The custom of eating milchige foods on Shavuos, however, remains cryptic and is not mentioned by many halachic sources, and that is why there are so many possible explanations. (Note that the Remo explains the basis for the custom with a partial conjecture, “and it seems to me that the reason is...”, rather than stating a definitive rationale, as this minhag is of unclear background.)

The most common ways that people fulfill the custom to eat dairy foods on Shavuos are by having milchige meals or by serving dairy “Mezonos” foods after making Kiddush on Shavuos morning. Each of these approaches needs a bit of halachic analysis.
Dairy Meals on Yom Tov
When the Beis Ha-Mikdash stood, the mitzvah of Simchas Yom Tov (Rejoicing on Yom Tov) was fulfilled by partaking of the Korbon Sh’lamim. However, when there is no Beis Ha-Mikdash, the mitzvah of Simchas Yom Tov is expressed in alternative forms. (See Pesachim 109a.)

The Beis Yosef and Shulchan Aruch (OC 529:2) opine that there is no mitzvah to eat meat on yom tov in the absence of the Beis Ha-Mikdash, whereas the Bach and others hold that one should eat meat, even though it is not from a Korbon Sh’lamim. The Mishnah Berurah concurs with the Bach. (See Bi’ur Halacha ibid.) The Bach and Mishnah Berurah seem to hold that one fulfills the mitzvah of Simchas Yom Tov even without eating meat, but that there is an enhancement of the mitzvah when meat is consumed.

The Rambam (Hil. Yom Tov 6:18) states that - in addition to eating the Korbon Sh’lamim - the mitzvah of Simchas Yom Tov is fulfilled by men partaking of meat and wine, women wearing fine clothing and jewelry, and children partaking of treats. Some poskim interpret the Rambam as mandating two levels of Simchas Yom Tov: an objective one, consisting of eating Sh’lamim, as well as a subjective level, such that all people should experience the simchah of the festival as they personally prefer. Accordingly, eating meat is just an illustration of what generally causes simcha, but there is no mitzvah to eat meat per se. Therefore, the mitzva of simchas yom tov can be fulfilled by engaging in any act that brings one to simcha. The Tur (OC 529) quotes the Rambam, and one can assume that he agrees with this interpretation of the Rambam’s position.

When applied to Shavuos, one who follows the Bach and Mishnah Berurah should ideally eat a fleishige meal rather than a milchige one on yom tov morning, although he nonetheless technically fufills the mitvzah of Simchas Yom Tov with a dairy se’udah so long as he enjoys it. One who goes according to the Rambam and Tur would be advised to eat whatever type of meal he most prefers. The issue here is that most people really prefer meat meals to dairy meals, and it would therefore be best for such people to consume a fleishige se’udah for yom tov. (According to the Rambam and Tur, if one likes chicken as much as beef, he can eat chicken as his main course, whereas the Bach and Mishnah Berurah seem to hold that beef is preferred, as they note the idea of simchah being identified with basar, meaning “meat” proper.)

One should consult his personal moreh hora’ah as to how to conduct himself.
Eating “Mezonos” After Kiddush
There is a fundamental principle of ”Ain Kiddush ela bim’kom se’udah” - “Kiddush may only be made at (the site of) the meal”. (Pesachim 101a, Rambam Hil. Shabbos 29:8, Shulchan Aruch OC 273:1) Some explain this to mean that Kiddush needs to function as an introduction to the se’udah. Regardless of the rationale, one who makes Kiddush without a meal (i.e. he does not eat a se’udah after Kiddush or he recites Kiddush in a location other than where he eats the meal) does not fulfill the mitzvah of Kiddush and must make Kiddush again when and where he eats.

The Tur and Shulchan Aruch (ibid. s. 5) quote the Ge’onim that one can fulfill the mitzvah of Kiddush without actually eating a full meal at the time and place that he makes Kiddush. Rather, posit the Ge’onim, a person can eat a mere k’zayis of bread or even drink a revi’is of wine as his Kiddush-time “meal”, so that he fulfills the requirement of Kiddush bim’kom se’udah. The Mogen Avrohom (ibid. s.k. 11) and Aruch Ha-Shulchan (ibid. s. 8) explain that, according to the Ge’onim, one can eat what we refer to as ”Mezonos” foods after Kiddush and satisfy the rule of “Ain Kiddush ela bim’kom se’udah”. This interpretation of the Ge’onim’s opinion has become widely accepted, and many poskim permit partaking of “Mezonos” foods after Kiddush and advise against satisfying the mitzvah by merely drinking a revi’is of wine. (See MB ibid. s.k. 25.)

The overall position of the Ge’onim is one of dispute, as the simple interpretation of ”Ain Kiddush ela bim’kom se’udah” is that one must actually have his se’udah - a full meal with bread - upon making Kiddush, and some therefore advise that one is best not relying on the Ge’onim’s approach. (See Aruch Ha-Shulchan and Bi’ur Halacha ibid.; Hag. Rabbi Akiva Eiger on Mogen Avrohom ibid. s.k. 10.) This is why some people do not eat at Kiddush receptions after shul, unless they wash and eat Lechem Mishneh. However, the more prevalent practice is to rely on the Ge’onim’s view and make Kiddush followed by cake or other “Mezonos” foods. (Some hold that if one makes Kiddush and then eats Mezonos foods, he must make Kiddush again later on at his actual se’udah.)

If one follows common custom (the opinion of the Ge’onim), it would seem that he can satisfy the minhag of consuming milchige food on Shavuos by eating cheesecake after Kiddush on Shavuos morning. However, it is not so simple.

The approach of the Ge’onim only postulates that Mezonos food eaten after Kiddush satisfy the requirement of Kiddush bim’kom se’udah when the amount of Mezonos food is at least a k’zayis. (See MB s.k. 21 on OC 273.) The problem is that many types of cheesecake have very little flour, and one does not typically consume a k’zayis of the dough or flour part of a slice of such cheesecake in the requisite period of k’dei achilas p’ras - “the time it takes to eat a piece of bread” (2-11 minutes; ask your Rav). Thus, cheesecake with minimal dough/flour content would not seem to qualify as the Mezonos food to eat after Kiddush.

So, too, even though the b’racha rishona for cake and pie is “Mezonos”, even when the majority of the cake or pie consists of filling or fruit rather than flour, there is an exception when the flour or dough part of these desserts serves merely to hold the filling or fruit in place and is not intended to provide flavor (OC 208:2). Some cheesecakes are virtually all cheese, and they have a paper-thin layer of tasteless dough which merely keeps the cheese in place. This situation would warrant reciting a “Shehakol” and would likely not enable one to consume the cheesecake directly after Kiddush. (See OC ibid. s. 9 and MB ibid. s.k. 45.)

Should one wish to have cheesecake after morning Kiddush, the solution would be to either purchase a cheesecake that has sufficient dough/flour (a k’zayis worth that will be consumed within the shiur of k’dei achilas p’ras), or to also eat a k’zayis of another type of Mezonos food (e.g. cookies, pastry or cake) making sure to have a k’zayis of the Mezonos food in a period of k’dei achilas p’ras, as above. In case one wants to consume a shehakol cheesecake, one should first eat a mezonos item right after the Kiddush prior to eating the shehakol cheesecake.
Eating meat after milk
What is the halacha if one makes Kiddush and eats milchige foods, planning to later eat a fleishige seudas yom tov? What if one eats a dairy yom tov seudah at midday and plans to eat a fleishige sholosh seudos later? How does one transition from milk to meat?

Although the details of this topic appear in in-depth articles on www.OU.org and www.OUKosher.org, here is a very brief review of the basics:

One should always wash his hands to remove any dairy residue on them. The poskim endorse this practice even if one’s hands appear to be fully clean.

One needs to cleanse and rinse his mouth. It does not matter which is done first, but both must be done.

The above pertains only to one who had a milchige meal and then wishes to eat ”meat” in the true sense of the word, such as beef, veal or venison. Poultry requires no washing of hands, cleansing or rinsing of the mouth when eaten after dairy foods.

If a person ate certain types of hard, aged cheese, the Ashkenazic practice is to wait before consuming meat for the same amount of time that one needs to wait after meat before consuming milk.

Is there any required waiting period if one eats dairy foods (other than hard, aged cheese) before partaking of meat? The Rishonim and poskim do not require one to wait at all, although the Zohar stipulates that one needs to wait an hour. Although halachically, one need not wait, many have the custom to wait for an hour or half an hour based on the Zohar.

May we be zocheh to be mekabel the Torah with full yir’ah, ahava and his’chadshus.