The holiday of Shabuot is one of the three "Regalim," and, as such, the Misva of "Ve’samahta Be’hagecha" (rejoicing on the holidays) applies. In fact, the Gemara in Masechet Pesahim (68b) comments that with regard to the holiday of Shabuot, all opinions agree that the observance must include an element of "Lachem" – physical enjoyment – because this is the day when the Torah was given. The Gemara relates that Rav Yosef would prepare an "Igla Tilta," an especially scrumptious meat delicacy, in honor of Shabuot, because, in his words, "If not for this day, there are many Yosef’s in the marketplace." Meaning, it is only because of the giving of the Torah on Shabuot that we are able to achieve spirituality, and this day therefore warrants festive celebration.
The Rambam writes in Hilchot Yom Tob (6:18) that the Misva of Simha (rejoicing) on Yom Tob requires partaking of meat and wine. In this context, "meat" refers specifically to red meat, and thus one must make a point of eating red meat on Shabuot. The primary Misva of Simha applies during the day, so one should have wine and meat – at least a Ke’zayit – during the daytime meal on Shabuot. It is proper to drink wine at night, as well, though on the first night on Shabuot one should minimize his wine consumption so he would be able to remain awake throughout the night. The daytime meat meal should preferably be eaten before Hatzot (midday), which this time of year is approximately 1pm.
Of course, as we know, there is a time-honored custom to eat dairy products on Shabuot, and "Minhag Yisrael Torah" – established customs are to be regarded like Torah law. However, this custom should not prevent one from fulfilling the Misva of eating meat. Therefore, some people have a dairy meal at night and a meat meal for lunch the next day. It should be noted that having a dairy meal on the first night of Shabuot might be a good idea in any event as the men who remain awake throughout the night might want to drink coffee with milk, so it would be advisable not to eat a meat meal that night. One can also fulfill the custom by eating something dairy, like cheesecake, in the morning after the prayer service, before lunch. Hacham Ben Sion Abba Shaul (Israel, 1923-1998) said that one can fulfill the custom with any dairy food, even an ice cream pop.
If one eats a dairy meal with bread in the morning, and then has a meat meal later, he must ensure not to use the same loaf of bread for both meals. If a loaf of bread was used at a dairy meal, there is a reasonable chance that it came in contact with dairy foods, in which case it may not be eaten with meat. One must therefore ensure to use a separate loaf for the meat meal. It should also be mentioned that when one buys Hallot in the supermarket, care should be taken to ensure they are wrapped in bags so that they do not come in direct contact with any dairy food. Sometimes, even packaged dairy foods such as yoghurt could have residue on the packaging, and if a Halla touches the packaging it might become dairy. One should therefore ensure that the Hallot are properly wrapped before they are placed in the cart.
In conclusion, Hacham Ben Sion Abba Shaul suggested a novel insight to explain the importance of eating meat on Shabuot. One of the reasons given for the custom to eat dairy foods on Shabuot (as cited by the Mishna Berura) is that Beneh Yisrael were unable to eat meat immediately after receiving the Torah. Once they received the Torah, they needed to learn all the rules for slaughtering animals and to prepare proper knives, and so on. They therefore had no choice but to eat dairy products, as they needed to eat before they were ready to prepare meat in accordance with Torah law. Hacham Ben Sion suggested that since that year Beneh Yisrael were unable to properly observe the Misva of Simhat Yom Tob, as they could not eat meat, we make a Tikkun (rectification) by ensuring to eat meat on Shabuot. Just as we remain awake throughout the night as a Tikkun for Beneh Yisrael’s having slept until the morning of Matan Torah, when they should have been awake eagerly anticipating their receiving the Torah, we similarly make a Tikkun for their not having eaten meat on Shabuot that year.
Summary: Although there is a custom to eat dairy foods on Shabuot, there is also a strict Halachic obligation to eat meat and drink wine on Shabuot, at least during the day. Therefore, one should fulfill the custom either by eating a dairy meal at night or by eating some dairy product in the morning, before lunch. One must ensure not to use the same loaf of bread at both a dairy meal and a meat meal.