Our practice is to read Megillat Esther twice on Purim, once at night and once in the morning (Shulchan Arukh, Orah Chayyim, 687:1). The source for this practice is from the Talmud (Megillah 4a), which quotes R. Yehoshua ben Levi and separately a Rabbi Helbo in the name of Ulla of Biri that a person is supposed to read the Megillah twice. Both Amoraim quote verses from Tehillim, which was written before Megillat Esther, to support their rulings but they do not provide a reason why the Megillah should be read twice. Every Shabbat we read the Torah once, why should the Megillah be read twice?
It
is highly likely that the initial practice was just to read the Megillah
once. The Mishnah (Megillah 1:1) records
that villagers would read the Megillah earlier than Purim, on the day of
assembly. The days of assembly were
market days when the villagers would come to the bigger towns for business, and
then they could hear the Megillah on these days even though it was not yet
Purim. Rashi (commentary on the Mishnah) explains that the villagers were illiterate
so they could not read on their own, and Chazal did not require the residents
of the villagers to make an extra trip to come to the bigger towns on Purim.
This law of reading the Megillah before Purim on the days of assembly is no longer applicable (Megillah 2a, Rambam, Laws of Megillah 1:9) but the law shows that the Megillah was only read once during the day. The residents of the villagers only came on the day of the assembly to hear the reading of the Megillah. They did not sleep over the night before and they did not come twice to the larger towns, once at night and once during the day.
In addition, the Mishnah (Megillah 2:5) records that the Megillah and other commandments can be done all day, and then the next Mishnah records two laws (reaping the omer and putting the fats on the altar) that can be done at night. If the Megillah was read at night, why did the second Mishnah not list reading the Megillah at night? It seems clear that in the times of the Mishnah, Megillat Esther was only read once during the day, and not at night. (This point has been made by Harav Shlomo HaCohen, 1828-1905, Vilna, Responsa Binyan Shlomo, 58, Hanoch Albeck, 1988, p.349, and Tabory, 1996, p.347. Tabory, footnote 76, quotes other sources arguing for and against this understanding. Also see Zevin, 1956, p.159, footnote 47, for more sources.)
A halakhic vestige of this practice of reading the Megillah only once in the daytime is the practice of Ashkenazim to recite the blessing shehecheyanu on the reading of the Megillah during the daytime, Rama 692:1. The Rambam (Laws of Megillah, Purim 1:3) writes that one does not recite shehecheyanu during the daytime since it is recited at night. This ruling is logical since we only recite shehecheyanu the first time when doing a commandment so it should only be recited once, and this is the ruling of the Shulchan Arukh, 692:1. However, Tosfot (Megillah 4a, Chayev) quotes R. Yitzhak that we recite the shehecheyanu even on the daytime reading since this is the main time for the reading. Similarly, the Tur (692) quotes Rabbenu Tam that the blessing is to be recited during the daytime and that his father, the Rosh, agreed with Rabbenu Tam. Also, the Maggid Mishnah (14th century, comments on the Rambam) writes that the custom in his time in Spain was to recite the shehecheyanu during the daytime.
This idea that the main reading of the Megillah is the daytime reading is most likely because in the time of the Mishnah, it was the only reading of the Megillah. Furthermore, even if the daytime is the main reading this still should not be enough to warrant reciting the shehecheyanu blessing twice. More likely, in the time of the Mishnah, the Megillah was only read once and then shehecheyanu was recited on that reading. Afterwards, in the time of the Talmud, when the night reading was added, some communities retained the old custom of reciting the blessing of shehecheyanu during the daytime.
Why in the time of the Talmud was it decided to read the Megillah twice? (Note this question still exists even if one wants to claim that initially the Megillah was read twice.) My guess is that this extra reading was instituted because of the practice of getting drunk on Purim.
The Talmud (Megillah 7b) quotes Rava that a person is to become so intoxicated on Purim that he cannot tell the difference between the good Mordechai and the evil Haman. The Talmud then relates an incident where Rabbah and R. Zeira had a Purim meal together and Rabbah was so drunk that he killed R. Zeira. However, a miracle happened the next day and R. Zeira came back to life. The next year, Rabbah again invited R. Zeira to have a Purim meal together, but R. Zeira declined since he said that miracles do not happen all the time. While clearly the story cannot be understood literally as R. Zeira was not resurrected, what is the message of this story?
R. Yosef Karo (1488-1573, Beit Yosef 695) quotes the Ran who quotes Rabbenu Efrayim that the point of the story of Rabbah and R. Zeira is to show that it is wrong to get drunk on Purim. The Bach (1561-1640, Krakow, 695) adds that this lesson is why the story is quoted immediately after the statement of Rava that a person should get drunk.
Other people (Zevin, 1956, p.203, quotes the Eshkol and the Prei Hadash) argue that the story of Rabbah and R. Zeira shows that people are supposed to get drunk. The logic is that if it was wrong to drink, then R. Zeira should have told Rabbah not to drink and then they could have their meal together without R. Zeira worrying that Rabbah would get drunk and kill him again. This counter-argument is not compelling. Clearly the practice of many people on Purim was to drink as indicated by Rava's statement and Rabbah's behavior, but the question is why were they drinking? Were they drinking because of a religions obligation or because this was the common practice to drink on Purim? If a person believes that it is wrong to drink on Purim, then he views the drinking as having no religious value and is simply part of the general culture of the people. It is very hard to stop people from doing something which they have grown up with as part of the general culture. Had R. Zeira told Rabbah not to drink, would Rabbah have listened to him? R. Zeira thought no since the culture was so strong to get drunk, and hence he declined the invitation to dine together a second time. (In my own life, I have seen that pointing out to people not to do something which is ingrained in their psyche is hopeless.)
The net result is that my understanding of the Talmud is like Rabbenu Efrayim and the Bach that the editors of tractate Megillah (Ravina and Rav Ashi?) viewed getting drunk as being horrible since it leads to terrible actions, and they made this point through the story of Rabbah and R. Zeira. Yet, one cannot deny that in the times of the Talmud people did get drunk on Purim, and this practice suggests two reasons why it was instituted to read the Megillah at night.
The first reason is that if the Megillah is only read during the daytime, then a person who gets drunk at night will not hear the reading in the morning. Thus, it was decided that it is better to at least read the Megillah at night than for it not to be read at all. It is true that the laws of drinking are always recorded within the laws of the meal of Purim and the meal is only during the day, but I have seen many times that people get drunk at night. Zevin (1956, p.205) quotes a piyyut in Machzor Vitry that reflects the massive drinking on the night of Purim, and refers to the night as the night of drunkenness.
A second reason for reading the Megillah at night is that the reading gives a religious framework to the drinking at night. This is especially important in reference to the drinking on Purim since it is not clear why it was the common practice to drink on Purim. It is very possible that this practice derives from the pagan festival of Dionysia. If there was no reading at night, then people would be drinking and celebrating but there would be no apparent religious basis for their actions. Thus, it was decided to read the Megillah at night, and then it can be claimed that the drinking is part of the celebration of Purim.