There is a well-known custom to eat dairy foods on Shavuot. The custom to eat dairy and honey on Shavuot is a custom that goes back 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.
The Rama (Orach Chayim 494:3) explains that the source of this custom is in commemoration of the Shtei HaLehem (two loaves), these two loaves of bread that were specifically offered on Shavuot in the times of the Mikdash. Just as we have two cooked food on Passover to symbolize the Korban Pesach and the Korban Hagiga, if we follow this same line of logic should not Shavuot too have a dairy meal and a meat meal to commemorate the two loaves of bread?
In light of this the custom was established by some 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.
Magen Avraham explains that the reason many eat dairy foods is because when the B'nei Yisrael received the Tora and learned all the laws of Kashrut, slaughtering and of koshering utensils, they needed to time to prepare meat in a kosher fashion and in the meantime they could only consume dairy foods.
Another source comes for this is that since we received Tora on the third of Sivan it is compared to milk as it written in: “Devash veHalav Tahat Leshonech” (Shir HaShirim 4:11).
Rav Kook presents also an explanation he says "that both milk and honey are foods which originate in something “impure” (not kosher). Honey is produced by bees, while milk is formed from blood. These foods involve the transformation of impure to pure and thus symbolize Tikkun Olam with their unique taste. Foods which symbolize transformation are appropriate to eat on the holiday celebrating the Matan Tora, as the Tora 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."
Another custom is to stay up all night is to make kiddush after Shaharit, eat dairy cakes, and then go to sleep. Wake up, then have a meat meal. Of utmost importance is the mitzva of simcha; all of these customs are meant to add joy to the holiday and honor to Matan Tora.
Conclusion: All of the above are customs and not laws they were upheld or not upheld for various reasons and various ways. For whatever the reason like all the beautiful customs of Yisrael they are meant to add joy to the holiday and bring honor to Matan Tora.
Chag Semeach Hakatzir V'Habikkurim