Reading the Megillah with a Minyan
When the megillah is read on Purim (the fourteenth of Adar in unwalled cities), or on Shushan Purim (the fifteenth of Adar in “walled cities”, such as Jerusalem), one should ideally read or listen to the megillah with a minyan. If, however, this is not possible, one may read the megillah by himself (Shulchan Aruch Orach Chaim 690:18). When, however, the megillah is not read in its proper time, it should only be read with a minyan. (Tractate Megillah 5a; Beit Yosef 690)
Why is it that on the actual day of Purim one may read the megillah by himself, whereas when reading it early it must be with a minyan?
The purpose of reading the megillah on Purim is in order to publicize the great miracle that
There is a dispute among the rabbinical authorities regarding what may be done when the megillah is read for Shushan Purim when it falls on Shabbat, and the megillah is read on Friday, the day before. According to Rashi’s reasoning, one would be allowed to read the megillah without a minyan since everyone reads the megillah on that day. However, the Ran maintains that in such a case one would not be able to read the megillah by himself, without a minyan.
The Turei Even explains the Ran as follows: When the megillah is read on Purim, because there are other celebratory activities performed on that day, such as the special Purim feast, giving food gifts to friends and charity to the poor — they add up to publicizing the Purim miracle, and there is therefore sufficient publication of the Purim miracle even if the megillah is read in private.
However, when the main celebrative activities (the Purim feast and the giving of food gifts to friends) are missing when the megillah is read the day before, reading the megillah without a minyan does not sufficiently publicize the miracle. In other words, when there is already a dominant public display of the Purim miracle via the other Purim mitzvot, one may read the megillah privately, but when the megillah is the main display of the holiday it must be read publicly in order to fulfill the requirement of publicizing the miracle.
Because there is a dispute regarding the this halacha, the Mishneh Berurah rules that in the event that one needs to read the megillah without a minyan when Shushan Purim is pushed back to Friday, he should do so without reciting a blessing (Mishneh Berurah 690:61; see Sha’arei Tzion for a different explanation of the rabbinical dispute).