The Mishnah Berurah (429:1) states that one should study and discuss each holiday in the thirty days leading up to it. Seudat Purim is one facet of Purim that is subject to a debate between the Talmud Bavli and the Talmud Yerushalmi. The Bavli states that one celebrating Purim on Shabbat should eat his Purim Seudah on Sunday rather than on Shabbat. Conversely, the Gemara Yerushalmi disagrees and asserts that one should eat his Purim Seudah on Shabbat. The generally accepted Halacha, offered by the Shulchan Aruch (O.C. 688:6), is that we rule in accordance with the Gemara Bavli, due to the concept of Halacha KeBatrai, which dictates that in an argument between the Bavli and the Yerushalmi, we follow the Bavli because it was complied after the Yerushalmi (see the comments of the Rosh to Chullin 2:6).
However, the Pri Chadash believes that ideally one should accomodate both opinions by eating a meal both on Shabbat and on Sunday. The question arises: since it is not very difficult to have two meals, one on Shabbat and one on Sunday, why did the Shulchan Aruch not offer this suggestion as well?
Perhaps the Shulchan Aruch understood that the Bavli had a fundamental problem with the Yerushalmi’s view of having a Purim Seudah on Shabbat and felt that not only is it not a Mitzvah to eat the Purim Seudah on Shabbat, but it is in fact prohibited. The reason for this could be due to the concept of Ein Mearvin Simchah BeSimchah, one should not combine two festivities together. This rule would dictate that it would be inappropriate to have a Seudat Purim, one festivity, on Shabbat, a festivity in its own right. Therefore, the Shulchan Aruch disagreed with the Pri Chadash and felt that it is not preferable to conduct the Purim Seudah on Shabbat, but it is in fact prohibited according to Halacha.
Outside of the confines of a walled city, this Halacha is very impractical, as Purim never coincides with Shabbat. However, within a walled city, such as Yerushalayim, residents will be faced this Purim with the decision of whether to follow the Shulchan Aruch or the Pri Chadash. Unfortunately, we no longer maintain Jewish residence in all of Yerushalayim and many of us do not live there anymore. Ideally, we will all be in Yerushalayim this Purim and be able to participate in deciding this complex Hachic issue.