I tried writing this post before, and Blogger ate it. I'll try to reconstruct what I can.
There is a tannaitic controversy about whether the book of Esther should be canonized. Some hold that remembering Amalek only belongs in Tanakh three times (in Exodus, Deuteronomy, and Samuel), while some hold that there's room for a fourth time. Anyway, I think that the various ways of parsing the texts on Amalek are just smoke and mirrors, hiding the real controversy. What is the book of Esther -- just a Persian-style royal comedy, or something with a deeper theological and religious significance?
The controversies about whether Kohelet and Shir Hashirim are sacred texts are also brought in; each of those books also stretches our conventional definitions of scripture. Ruth is mentioned, but no one actually takes the position that it doesn't belong in the Bible. The fifth megillah (Eicha) isn't even mentioned.
In any case, everyone seems to agree that the book of Esther came from the Holy Spirit (really! ruach hakodesh), but some think that it was given to be recited and not written down. (Does this make it Oral Torah? Fitting, for a biblical book that gives us a "rabbinic" holiday.)
Everyone has his own proof for why Esther must have been said by the Holy Spirit. They're all fascinating as long as you don't think too hard about them and realize how similar they sound to sketchy contemporary attempts to prove divine authorship of texts from within the texts (a classic example of "begging the question"). Ravina (or was it Rava?) is able to shoot down each of these proofs, except the final one by Shmuel. One proof comes from the line "Haman said in his heart" -- how could the author have known what Haman was thinking? (Ravina/Rava responds that Haman wasn't exactly subtle about what was going through his head, so it didn't take a telepath to figure that out.) "Esther found favor in the eyes of all who saw her" - again, how could the author know? (Response: there is a midrash that each person saw Esther as a member of his/her own nation.) "The thing became known to Mordechai." (We'll get a midrash to explain this one away on 13a. We decided to maintain the suspense and not read ahead.) "They didn't lay their hands on the spoils." No one? (They sent a telegram to Mordechai and Esther saying that they weren't taking anything.) Finally, Shmuel uses "established and accepted" (Esther 9:27) -- this means that they established above what had been accepted below. In other words, God itself established Purim when the Jews had accepted it. Only God (and apparently Shmuel) could have known what God was doing, so the divine authorship of Esther stands. Really finally, Rav Yosef (and a copycat) uses "These days of Purim shall never depart from the Jews" -- the author must have seen into the future! MAK points out that Rav Yosef reads this as a positive statement when most readers would read it as a normative statement.
Next, we get into the other mitzvot of Purim: mishloach manot, matanot la'evyonim, and se'udah. "Mishloach manot (plural) ish l'rei'eihu (singular)", so the minimum requirement is two gifts to one person. "Matanot (plural) la'evyonim (plural)", so the minimum is two gifts to two people. Rabbi Yehudah Nesiah (not to be confused with Rabbi Yehudah haNasi) sent Rabbi Osha'ya a steak and a bottle of wine. Rabbi Osha'ya replied "You have fulfilled mishloach manot." This is where my sarcasm detector fails me. Was he just being straightforward and stating that this was in fact two items, or was this a tongue-in-cheek response to something that by our cultural standards would be far and above the mishloach manot that one expects?
Rabbah sent dates and toasted flour, and Abaye warned that this would betray his low-class roots. The wealthier Marei bar Mar sent back a spice (possibly ginger) and pepper, and Abaye warned that he was responding to something sweet with something bitter.
This leads into more stories about food which just weren't very good. The food apparently was good, but not the stories. The moral: When you're poor, you forget just how hungry you are. The other moral: There's always room for Jell-O.
And now the most famous line, quoted even in letters to the Harvard Crimson. Rava said: A person is required l'vasomei on Purim until s/he does not know the difference between "Cursed is Haman" and "Blessed is Mordechai". I left l'vasomei untranslated, because it's not entirely clear what it means. It's usually translated as "to be intoxicated", following Rashi's commentary ("to get drunk with wine"), but that's not obvious -- the Korban Netanel (one of the guys in the back) says that he wouldn't have known what it meant if it weren't for Rashi. (The Korban Netanel also says that the "until" in Rava's statement is "up to, but not including".) Mar Gavriel suggests that it actually means "to get others drunk", not to get drunk yourself.
Whatever Rava's statement really means, it is quickly shot down! Many people who seek to use Rava's statement to justify their drunken excesses stop reading here, but let's keep going: Rabbah and Rabbi Zeira had a Purim feast together. They got drunk. Rabbah got up and slaughtered Rabbi Zeira. The next day, he prayed for mercy, and Rabbi Zeira was revived. The following year, he invited him to another Purim feast, but Rabbi Zeira responded "A miracle doesn't happen every time!"
I hold by Rabbeinu Ephraim (quoted by the Ran), who says that this story constitutes a complete refutation of Rava's statement, so it is not advisable to follow Rava and get drunk on Purim. NAF says "If we all followed Rabbeinu Ephraim, we'd be drinking virgin daiquiris every night!"
Finally, the Purim feast must happen during the day.