A person who will be traveling on Purim and will not have access to a Megilah may read the Megilah before he leaves, starting from the eleventh day of Adar. If he will be traveling before the eleventh, some hold that he can even read the Megilah from Rosh Chodesh. Since the Megilah is being read before the proper time, he must read with a minyan for pirsumei nisa. The minyan in this case can be comprised of minors. A brocha is not recited on this reading. Under all circumstances, Mishloach Manos, Matanos L’evyonim and Seudas Purim must take place on Purim itself. If the traveler ends up procuring a Megilah on Purim, he should read it again with a brocha.
[שו"ע תרפח, ז, משנ"ב כ ו־כב, שעה"צ לג, וביה"ל ד"ה יקראנה]