Megillah 5a: Rav said: “When Purim is observed in its regular time, an individual alone may read the Megillah, but when it is observed outside of its regular time, such as when it falls on Shabbos and is pushed back to Friday, one must read the Megillah with a minyan.”
But does this not contradict the following statement of Rav: “When Purim falls on Shabbos, Friday is the regular time.” Actually, Shabbos is the regular time. (Rashi: Because the Anshei Knesses Hagedolah ordained Purim on Shabbos, and it was only a later generation of sages who made the decree not to read the Megillah on Shabbos.) So when it says Friday is the regular time, doesn’t it mean that one does not need a minyan on Friday? – No. it just means that Purim is not pushed back to Thursday.
This Gemara is discussing the case when the 14th of Adar fell on Shabbos, but the Mishnah Berurah (690:61) extends the rule (that when it is not the regular time, we need a minyan) to the case when the 15th of Adar falls on Shabbos and the Megillah is read in a walled city on Friday.
מגילה ה ע”א. אמר רב: מגילה, בזמנה ־ קורין אותה אפילו ביחיד, שלא בזמנה ־ בעשרה… ומי אמר רב הכי? והאמר רב יהודה בריה דרב שמואל בר שילת משמיה דרב: פורים שחל להיות בשבת ־ ערב שבת זמנם. ־ ערב שבת זמנם? והא שבת זמנם הואִ! אלא לאו הכי קאמר: שלא בזמנם ־ כזמנם, מה זמנם אפילו ביחיד, אף שלא בזמנם ־ אפילו ביחידִ ־ לא… לאפוקי מדרבי וכו’.
שו”ע או”ח תר”צ. מגילה בי”ד ובט”ו צריך לחזור אחר עשרה ואם אי אפשר בעשרה קורים אותה ביחיד. וכתב המ”ב: דהיכא דמתרמי יום ט”ו בשבת דצריכין המוקפין להקדים ולקרותו בע”ש בודאי צריך לקבץ עשרה לקריאתו, ובזה עוד חמיר יותר דאי ליכא עשרה לא יברכו המוקפין עליה.
Once, in Yerushalayim in a year when the 15th of Adar fell on Shabbos, and the Megillah was read on Friday, the baal korei in one particular shul fell suddenly ill after the reading, and died on Friday afternoon. Rabbi Shlomo Zalman Auerbach paskened that all those who had heard the Megillah from him would have to hear it again. The reason is that the real time for Krias Hamegillah is Shabbos, and Friday is just a replacement for Shabbos. This baal korei, since he did not live to see Shabbos, was not obligated in the mitzvah, and thus was incapable of being “motzi” the listeners in their obligation.
[The Chazon Ish disagreed with the Mishnah Berurah’s contention that when a walled city pushes back the reading to Friday, they must have a minyan. He argues that since Friday is the day everyone in the world is reading the Megillah, there is enough pirsumei nisa and one doesn’t need a minyan.
But it would seem that in R’ Shlomo Zalman’s case, even the Chazon Ish would agree that the listeners did not fulfill their obligation with the now-deceased baal korei. For even the Chazon Ish agrees that Shabbos is the real time originally ordained for the reading, and Friday is just a replacement for Shabbos. The fact that this replacement time happens to be the time when the rest of the world is laining does not change the fact that it is, after all, just a replacement. And someone who is not obligated in the original day is not obligated in the replacement either.]
Source: Rabbi Aryeh Golovenzitz