When the fourteenth of Adar falls on Friday, which means that the fifteenth (the day Purim is usually observed in Yerushalayim) falls on Shabbos. This causes changes to many Purim observances, and we call the result "Purim Meshulash."
Below is a simplified guide to the practices of Purim Meshulash. THURSDAY
Thursday, Taanis Esther is the same as every year. We donate Machatzis HaShekel at mincha, as in other years.
One important halacha is harder to observe this year – not eating before hearing the megillah. In most years this is not a serious issue for Yerushalayim residents, since one may eat the entire day of the fourteenth. However, this year, when we read megillah immediately after Taanis Esther, this requirement can become a hardship, particularly for those who can only hear the megillah at a later reading. If necessary, one may eat fruit, vegetables, dairy items, or rice before hearing the megillah, but one may eat only a very small amount of pasta, pastry or bread. If one might become weak or ill, one may eat a regular meal.
One may drink before hearing megillah reading. Of course, if there is any question, ask a shaylah. THURSDAY NIGHT
We do not say Al Hanissim in davening or bensching on Thursday night or Friday. (If someone said Al Hanissim by mistake, he does NOT repeat davening or bensching.) We hear megillah on both Thursday night and Friday morning. One should wear Shabbos clothes to the Megillah reading and all day both Friday and Sunday.
An important word about Megillah reading this year: Because we hear megillah this year on a day that is not Purim for us, one must hear megillah with a MINYAN. A woman who cannot hear megillah reading in shul should strive to hear megillah in a place where there are ten women. (The background behind this shaylah is more complicated than I can explain here, but for those who are interested I can send a more extensive explanation.) Someone who is hospitalized or who cannot possibly hear megillah together with ten other people fulfills the mitzvah by hearing a private reading. FRIDAY MORNING
Shacharis is without keriyas haTorah, tachanun, or lamnatzei’ach. Megillah is read. When the shehechiyanu is recited on the megillah, one should have in mind to include the mitzvos of matanos la’evyonim, shalach manos, and seudah with the shehechiyanu, even though we will not perform shalach manos and the seudah until Sunday.
We distribute Matanos La’evyonim on Friday.
According to the Chazon Ish and some other poskim, we should distribute shalach manos on Friday. (As mentioned above, most poskim contend that we give shalach manos on Sunday.) It is a hiddur to give one person shalach manos on Friday to fulfill the minimal requirement of the mitzvah according to the Chazon Ish. SHABBOS
We recite Al Hanissim during all davenings and benschings. If one forgets to say it, one does NOT repeat davening or bensching. However if one remembers before taking three steps back after davening, one should say Al Hanissim in Elokai Netzor, and if one remembers before completing bensching, once recites it in the Harachamans. In the latter case, one should introduce the "Bimei Mordechai" with the following words "Harachamon Hu yaaseh lanu nissim viniflaos kaasher asah laavoseinu bayomim haheim bazman hazeh). We "lein" the Purim keriyas haTorah as maftir, and the haftarah is the same that was recited last Shabbos, Parshas Zachor.
Some poskim hold that the mitzvos of seudah and shalach manos are observed on Shabbos. Although this opinion is not generally accepted, the custom is to serve an extra side course and some extra wine at the Shabbos meal in honor of Purim and to have guests at the table to fulfill shalach manos. (Alternatively, one shares some of one’s Shabbos meal with a neighbor). SUNDAY
This is when we fulfill the mitzvos of the seudah and shalach manos, and the general simcha of Purim, costumes and all. Nevertheless, we do not say Al Hanissim in either davening or bensching. We also do not say Tachanun or Lamnatzei’ach, nor is there keriyas haTorah. One should wear Shabbos clothes (unless one chooses to wear a costume). COMPLICATING SHAYLOS
Because of reasons too detailed to discuss here, someone who intends to spend either Thursday or Friday night out-of-town should ask a shaylah to find out how these trips impact halachically. Also, someone from out of town who intends to be in Yerushalayim for one of the megillah readings should ask a shaylah first.
The pasuk tells us that the events of Purim resulted in "Vi’nahafoch hu," "It turned around" – we turned the tables on our enemies and vanquished them on the day declared for our extermination. A different type of vi’nahafoch hu occurs on Purim Meshulash -- Many observances become turned upside down, creating a very different, and very enjoyable Purim experience.
This Shiur is published also at Rabbi Kaganof's site