On the first day of Sukkot (if it does not fall on Shabbat), we recite two Berachot when taking the Lulab: the Beracha of "Al Netilat Lulab," which we recite each day of Sukkot when taking the Lulab; and "She’hehiyanu." We recite "She’hehiyanu" on the first day just as we recite this Beracha over any Misva which is observed at a specific time during the year.
If a person forgot to recite "She’hehiyanu" when taking the Lulab on the first day of Sukkot, and he realizes his mistake while he is still involved in the Misva (the waving and the Hoshanot), then he may still recite it at that time. The more complicated question arises if the person did not realize his mistake until after he finished with the Misva on the first day. Should he recite "She’hehiyanu" when he takes the Lulab on the second day, or is it too late to recite this Beracha over the Lulab that year?
One might, at first glance, argue that since the Misva of Lulav applies on the level of Torah obligation only on the first day of Sukkot, after which it applies only by force of Rabbinic enactment, "She’hehiyanu" cannot be recited after the first day. In truth, however, this argument is incorrect. We recite the Beracha of "She’hehiyanu" even over Misvot which were legislated by Hazal, such as reading the Megila and Hanukah candle lighting. Therefore, the fact that the Misva of Lulab is required only on the level of Rabbinic enactment, as opposed to the level of Torah obligation, is not a reason not to recite "She’hehiyanu" over this Misva on the second day.
Still, one might argue that "She’hehiyanu" can no longer be recited since the person had already fulfilled the Misva the previous day, and thus the Misva is no longer new and does not warrant the recitation of "She’hehiyanu." Indeed, the Peri Megadim (Rav Yosef Teomim, 1727-1792), as cited by the Hafetz Haim (Rav Yisrael Meir Kagan of Radn, 1839-1933) in Sha’ar Ha’siyun (662), ruled that one may not recite "She’hehiyanu" over the Lulab after the first day.
The consensus among the Halachic authorities, however, does not accept this line of reasoning. Numerous Poskim, including the Magen Abraham (662:1), Lebush (644:1) and Hayeh Adam (148:10), rule that one who did not recite "She’hehiyanu" over the Lulab on the first day recites the Beracha the second day. This is the conclusion reached by Rabbi Moshe Halevi (Israel, 1961-2000), in his work Birkat Hashem (vol. 4, p. 108).
It should be noted that if somebody did not recite "She’hehiyanu" over the Lulab on either of the first two days of Sukkot, then he recites it when he takes the Lulab on the third, and if he did not remember on the third day, then on the fourth. Even if one did not recite "She’hehiyanu" the first six days of Sukkot, and he remembers only on the seventh day (Hoshana Rabba), he recites the Beracha when taking the Lulab on that day.
Summary: If one forgot to recite "She’hehiyanu" over the Lulab on the first day of Sukkot, then he recites it when he takes the Lulab on the second day, and if he forgets again on the second day, then he recites it on the third day, or on the first day he remembers.
