Machtzit Hashekel (Orach Chayim ad loc.) draws our attention to the above Gemara as a source not only for the obligation to make the se’udat Purim during the day but also as a source that there is somewhat of a mitzvah to make a se’udah at night as well. As the Gemara relates: “R. Ashi, who was sitting before R. Kahana late in the day [of Purim], asked: ‘Why did the rabbis absent themselves from the study hall today?’ The other responded: ‘Are you not aware of Rabba’s ruling that the se’udah must be during the day?’” The fact, though, that R. Ashi even asked such a question serves as proof that there is somewhat of a mitzvah to feast at night as well as it is obvious that R. Ashi’s assumption was that the rabbis had already made their se’udah the night before.
The Mishnah Berurah (ad loc. sk3) notes that even should Purim fall on Sunday, and a person had a full third Shabbat meal, he nevertheless should also feast somewhat again at night, but not to the same degree as during the day of Purim.
The Rema (ad loc. Orach Chayim 695:2) states that our custom is to make the se’udah after Mincha, and Ma’ariv should be prayed when it is surely night. Thus, the se’udah should properly be between Mincha and Ma’ariv, i.e., late afternoon. If one starts when it is practically night, the bulk of the se’udah will be on the eve of the 15th of Adar – Shushan Purim.
He notes further that when Purim falls on the eve of the Sabbath, the se’udah should be made in the morning in deference to the Shabbat meal in the evening. The Mishnah Berurah explains that the reason we don’t usually make the se’udah in the morning (when it is not a Friday) is simply because we are busily engaged in the mitzvot of the day – megillah, charity, mishloach manot – and these can take until Mincha to complete.
The Rema cites Terumat Hadeshen who offers a leniency regarding the timing of the se’udah. “And if one wishes to always make the se’udah [i.e., when it is not Friday] in the morning, one may do so.”
Thus, if for some reason a person must travel on Purim afternoon and is not sure if he will reach his destination in time to make the se’udah there, it seems he would have the option (though not the optimum means) of fulfilling the obligation by having the se’udah, an elaborate meal of meat and wine, in the morning. A simple breakfast of cereal and milk would obviously not suffice.
While there is no restriction on traveling on Purim, one should always keep in mind the miraculous turn of events that G-d wrought for our people at that time – for all time and pray that we merit the ultimate redemption speedily in our days.