In a year when Purim falls on Mosa’eh Shabbat, an interesting question arises concerning those who normally observe Shabbat until the time when it ends according to the view of Rabbenu Tam – seventy-two minutes after sundown. If the congregation ends Shabbat in accordance with the more widely-accepted view, forty-two minutes after sundown, and they read the Megila at that point, do those who normally wait until the time of Rabbenu Tam fulfill their obligation? According to their position, it is still Shabbat, and thus Purim has not yet begun. Seemingly, then, it is too early for them to fulfill the Misva of Megila reading.
Rabbi Mazuz rules that people who follow Rabbenu Tam’s view of the end of Shabbat may, indeed, fulfill their obligation by hearing the Megila reading before the time of Rabbenu Tam. He notes that the Shulhan Aruch (Orah Haim 692:4) cites a view allowing one to read the Megila as early as Pelag Ha’minha (approximately one and a quarter hours before sundown). This view is also cited in the Kaf Ha’haim (692:31). According to this view, it is certainly acceptable to hear the Megila forty-two minutes after sundown, even if one normally follows Rabbenu Tam’s opinion that Shabbat ends a half-hour after that point, since it is certainly past Pelag Ha’minha, even according to Rabbenu Tam. And although the Sages enacted a prohibition against reading the Megila on Shabbat, this applies only to reading the Megila, not hearing the Megila. The one who reads the Megila in the synagogue is following the view that Shabbat has already ended, and those who follow Rabbenu Tam’s view are only listening, which is not included in the prohibition against reading the Megila on Shabbat. Moreover, even according to Rabbenu Tam, forty-two minutes after sunset is within the period of Ben Ha’shemashot (the period between sunset and nightfall), and it is quite likely that the Sages did not extend their enactment to this period, and that it is thus permissible to read a Megila during Ben Ha’shemashot.
Furthermore, most people who follow Rabbenu Tam’s position concerning the end of Shabbat do so only as a "Humra," a measure of added stringency, while acknowledging that Halacha in truth follows the view of the Ge’onim, that Shabbat ends forty-two minutes after sundown. And many people who follow Rabbenu Tam’s view refrain during this period only from acts that are prohibited on Shabbat by Torah law (as opposed to by force of Rabbinic enactment). They would not go so far as to insist on waiting until seventy-two minutes past sundown before hearing the Megila reading.
Therefore, for all these reasons, Rabbi Mazuz rules that those who follow Rabbenu Tam’s position may fulfill their obligation of Megila reading by hearing the reading on Mosa’eh Shabbat before the time of Rabbenu Tam. However, people who follow Rabbenu Tam’s position should not read the Megila before the time of Rabbenu Tam; they may hear the Megila, but should not read it themselves until Shabbat ends according to Rabbenu Tam’s view.
Summary: When Purim falls on Mosa’eh Shabbat, and the congregation begins reading the Megila forty-two minutes after sundown, which is when Shabbat ends according to the commonly accepted view, even those who follow the later time for the end of Shabbat (the time of Rabbenu Tam) fulfill the Misva by hearing the reading.
