The Gemara in Shabbos 117b discusses what to do in a case where a fire is devastating one’s home on Shabbos. It would be permissible to take enough food for three meals out of the burning house and bring it into his courtyard or “karmelis.” The Gemara explains that if the fire would happen at night, before any meals were eaten, he may take out food for three meals. If the fire would happen in the morning, he would be allowed to take out food for two meals. If the fire would start at Mincha, he would only be allowed to take out food for one meal.

The Rambam in Hilchos Shabbos 30:9 says that everyone is required to eat three meals on Shabbos; one meal in the evening, one in the morning, and one at Mincha time. The Maggid Mishna explains the Rambam; it is a mitzvah for the three meals to be eaten at those times. It is a machlokes among the Geonim whether eating the meals at the proper time is a chiyuv or just the normal and regular time to do the mitzvah. The Bahag 16:3 writes that one is yotzai the mitzvah even if he stops in middle of the morning meal, says Birkas Hamazon, then washes and eats Hamotzi again. The Ramban and Rashba say that based on this opinion, when it comes to saving items from a fire, where the Gemara differentiated on the timing, it may change what is permissible. The timing of the meals is only a recommendation; one may actually eat his meals later or earlier. Therefore, in the case of a fire, he may save food from the fire depending upon how many meals he still did not eat. On a different note, the Sefer Ha’itim 193 says that it is a mitzvah to eat the meals at the proper time.

The Aruch Hashulchan 288:2 paskens that the morning meal must take place before chatzos. He explains this by saying; just like the third meal needs to be after chatzos, the morning meal needs to be before chatzos. The reason for this is that we learn from the pesukim that say the word hayom three times that we eat three meals and each one has to take place at a distinct time.

The Bach 334 says that if the fire took place after chatzos, one may only save food for one meal since the morning meal needs to be eaten before chatzos. The Maamer Mordechai s.k.1 disagrees with this position and says that the reason why one needs to eat his morning meal before chatzos is not because that is the time for the meal. It is just due to the fact that one may not fast on Shabbos; therefore, the person must eat before chatzos. If a person were not fasting since he ate or drank before chatzos, he would definitely be yotzei if he eats both day meals after chatzos.

The Mishna Berura brings down the Pri Megadim who says one may save food for two meals even after chatzos. It would seem that the Mishna Berurah holds that one may eat the morning meal after chatzos. We could differentiate that the real mitzvah is before chatzos, but there is an inyan of tashlumin for the morning meal. Therefore, if one did not eat before chatzos, and then had a fire in his home, he would be permitted to save food for two meals because he had not yet eaten the morning meal.

I suppose this is why we see people eat from a Kiddush like they are on fire; they are trying to make sure they don’t fast past chatzos.