I meant to post this on Friday but did not have time... better late thane never. The Yerushalmi writes that if Purim falls on Shabbos, seudas Purim is pushed off and held after Shabbos rather than earlier. This creates the phenomenon known as "Purim meshulash". If you are in Yerushalyim this year for Purim, you will hear megillah and do matanos l'evyonim on Friday, then have Shabbos, and have your Purim seudah on Sunday, creating a three day celebration.
Why, asks the Yerushalmi, is the Purim seudah not held on Shabbos? The Yerushalmi answers by quoting the pasuk from the megillah, "la'asos oso y'mei mishteh v'simcha", that Purim involves turning an ordinary day into a special day of simcha, as opposed to Shabbos, which is already fixed since creation as a day of simcha. You can't mix celebrations of G-d-imosed simcha and man-made simcha on the same day. The fact that we never find such a derasha in the Bavli already should make us suspicious that perhaps this Yerushalmi is not unanimously accepted, and sure enough, from Rishonim to Achronim, there is debate as to whether this conclusion is accepted l'halacha. Maybe more on that later in the week.
Even though this Yerushalmi is much discussed in the context of Purim, it actually applies to a halacha that came up this Shabbos, which coincided with Rosh Chodesh. The Shulchan Aruch writes (O.C. 419) that there is an obligation to mark Rosh Chodesh with a special seudas Rosh Chodesh (this halacha is not too popular in practice; see the Aruch haShulchan). We don't find a clear statement in the Bavli that such an obligation exists, but the Taz points to the same Yerushalmi above as a source. The Yerushlami writes that not only seudas Purim, but also seudas Rosh Chodesh is delayed until after Shabbos. QED from the fact that the two seudos are discussed in the same context that just like there is an obligation for a seudah on Purim, there is a similar seudah obligation on Rosh Chodesh. The MG"A writes that on Shabbos one needs to find some way to add something to one's seudah to make it more special and indicate that it includes this added kiyum of seudas Rosh Chodesh. However, if one accepts the Yerushalmi at face value, then in reality the seudas Rosh Chodesh cannot be fulfilled on Shabbos and needs to be celebrated after Shabbos. The Mishna Berura (Sha'ar haTziyun #5) quotes R' Ya'akov Emden's psak, based on the Yerushalmi, that one can fulfill the seudas Rosh Chodesh by having a special melaveh malka, even though Motzei Shabbos is not longer Rosh Chodesh.