The Unique Status of Ta'anit Esther
Generally, when a fast day occurs on Shabbat, we delay its observance until the following Sunday. This scenario is familiar primarily regarding Shiva Asar Be-Tammuz and Tisha Be-Av, each of which may occur on Shabbat. However, when Ta'anit Esther – the 13th of Adar - coincides with Shabbat, it is prescheduled on the proximate Thursday.
The Rav zt"l suggested that Ta'anit Esther is fundamentally different than the four other fasts: Asara Be-Tevet, Shiva Asar Be-Tammuz, Tzom Gedalya and Tisha Be-Av. The latter fasts commemorate national calamities relating to the destruction of the Temple. Since they recall disasters, their commemoration cannot be prescheduled, in light of the concept that "we do not accelerate tragic commemorations" ("akdumei puranuta lo makdaminan" – see Megilla 5a). Unlike these fast days, Ta'anit Esther demands that we undergo the "complete" Purim experience, including the fast days proclaimed in response to the national threat. Specifically, the unique demand of pirsumei nisa (publicizing the miracle) obligates us to publicize God's receptivity to our fervent prayers as the source of our rescue. Since Haman's threat ultimately was removed, Ta'anit Esther is not a day of sadness, but rather a prelude to the great joy which Purim evokes. As it commemorates no tragedy, it can be rescheduled to Thursday (see especially the She'iltot, Vayakhel 20).
There are several additional indicators that Ta'anit Esther is, indeed, a unique fast. Firstly, the Megilla itself appears to address the fast as part of the Purim celebration, when it describes the Jewish people accepting the Purim traditions "as they accepted the days of fasting" ("ka'asher kiyemu al nafsham ve-al zar'am divrei ha-tzomot ve-za'akatam," 9:31).
In fact, the Ra'avad cites this verse as part of his answer to a famous question about the validity of fasting at all on the 13th of Adar. Purim itself - aside from its status as a holiday - also belonged to the series of national holidays enshrined in Megillat Ta'anit. These days commemorated significant national events – on a much smaller scale than Purim and Chanuka. To commemorate these secondary but important events, no fasts or eulogies could be held on any of these national holidays. To lend these days greater authority, fasts and eulogies were prohibited also on the day prior and subsequent to these holidays. Consequently, the day before Purim (the 13th) should be a day on which a fast cannot be proclaimed, and Ta'anit Esther thus appears illegal! The Ra'avad responded that the fast of the 13th of Adar differs from ordinary fast days, in that it serves to commemorate the miracle and is alluded to in the aforementioned verse - "divrei ha-tzomot ve-za'akatam."
The scheduling of Ta'anit Esther actually suggests its nature as an integral component of the Purim festivities. Historically, the drama surrounding the entrapment of Haman – prefaced by three days of fasting - occurred during Nissan. If the fast commemorated that potential tragedy, it should have occurred during Nissan. Shiva Asar Be-Tammuz, for example, commemorates that calamity by fasting on the day during which those actual events occurred. The scheduling of Ta'anit Esther during Adar, immediately prior to Purim, implies its essence: to recreate a complete Purim exhilaration. The relief of Purim's miracle can only be appreciated by a prior simulation of the tension and fasts that were so pivotal to the miracle itself.
One might question the exact nature of the fast based on the above-stated deviations. The Rambam (Hilkhot Ta'aniyot 1:14) asserts that a fast day should be experienced without personal luxury, frivolity, or excess joy; a person should express on fast days feelings of anxiety and lament ("do'eg ve-onen"). Is such a mentality necessary or even appropriate on Ta'anit Esther? If, indeed, this fast launches the Purim festivities, then the halakha mentioned by the Rambam would not apply to Ta'anit Esther.
Interestingly, the Rema rules (Orach Chayim 686) that Ta'anit Esther is the most lenient of all fasts, and can be suspended for those who experience excessive discomfort. Would this be reflective of Ta'anit Esther's extraordinary status? Had it been commemorative of a national tragedy, we might have been less willing to exhibit leniency for sufferers. Since it participates in the Purim celebration, it might be waived in cases of particular discomfort.
The Rav zt"l posed an additional question that clearly stems from the previous one. The Rambam (Hilkhot Ta'aniyot 5:9) claims that the fasts will be abolished during the Messianic era; the tragedies which they represent will no longer be commemorated during the era of final redemption. Will Ta'anit Esther face a similar fate, or will it be retained? A different comment by the Rambam (Hilkhot Megilla 2:18) asserts that "even though the memory of the tragedies will fade, the DAYS of Purim will not be rescinded." When the Rambam mentions the DAYS of Purim, might he be including Ta'anit Esther as part of this experience, sparing it the fate of the classic fast days in the Messianic era? The Brisker Rav did, in fact, suggest this reading of the Rambam, further confirming our view of Ta'anit Esther as integral to the celebration of Purim itself.
An interesting responsum of the Geonim, however, might call this view into question. An opinion is cited demanding that we fast on the 13th of Adar during Adar I of a leap year. Even though Purim will not be celebrated during Adar I, this position maintained, the fast is still held. Would this not indicate that the fast is NOT integral to the celebration of Purim, since it is held during a month in which the Purim celebration does not occur? Of course, this position must be inspected in light of the general status of Adar I during a leap year. Is it truly a month entirely bereft of Purim celebrations, or are some Purim aspects allocated to it as well? (The Ran cites an opinion requiring that we actually conduct a Purim se'uda on the 14th of Adar I.)
This view of Ta'anit Esther might explain an intriguing gemara (Megilla 2a). Attempting to justify the multiple options for reading the Megilla (under certain circumstances, the megilla is read as early as the 11th of Adar), the gemara cites the word "bi-zemaneihem" (during their periods), which suggests multiple dates of reading. Recognizing that this phrase adds at least two additional options to the base options of the 14th (for regular cities) and 15th (for walled cities), the gemara questions that perhaps only the 13th and 12th should be added. How, then, can we justify reading the megilla as early as the 11th? The gemara responds that the 13th does not require a source to warrant Megilla reading, since it is "zeman kehilla la-kol," a day of public gathering. Though Rashi offers a dissenting opinion, most Rishonim interpret this concept as Rabbenu Tam did: since everyone gathers on the 13th of Adar for the fast, it is an appropriate day for Megilla reading, even without an overt textual precedent.
At first glance, we might question the nature of this self-evident warrant. The fact that people gather to conduct a fast does not necessarily mandate Megilla reading, unless we can find textual license. But if, as stated earlier, we view the fast as an incorporated element of the Purim festivities, or, as we suggested in the Rambam, as part of "yemei ha-Purim" (the days of Purim), we can easily understand Rabbenu Tam's argument. Viewing Ta'anit Esther as part of the Purim process, we do not require an additional verse to mandate possible Megilla reading on this day.