Starting from the Arvit prayer on the first night of Chanukah (this year, 5779, starting from tonight, Sunday night) “Al Ha’Nissim” is added in the Amida in the middle of the Blessing of Thanksgiving (“Modim Anachnu Lach etc.) as it is printed in all Siddurim.
Even if most of the congregation has not yet lit Chanukah candles, they still recite Al Ha’Nissim, for Al Ha’Nissim is not contingent upon the Mitzvah of lighting Chanukah candles, especially since in Al Ha’Nissim we praise Hashem for the miraculous victory over the wicked Greeks and not only about the miracle of the Menorah. Therefore, even if Chanukah candles were not lit yet, one would still add Al Ha’Nissim into the prayer. [On a year where Chanukah begins on Shabbat night, Al Ha’Nissim is not recited in the Mincha prayer of Friday afternoon. However, since this year, 5779, this is not the case, we will not discuss this matter at length.]
Similarly, one must add Al Ha’Nissim in all prayers during Chanukah including Shacharit, Mincha, Arvit and Mussaf of Shabbat Chanukah and Rosh Chodesh Tevet.
One Who Forgets to Add “Al Ha’Nissim”
If one forgets to add Al Ha’Nissim the Halacha is as follows: If one remembers that one forgot to include Al Ha’Nissim before one mentions Hashem’s name at the end of the blessing, i.e. “Baruch Ata Hashem Hatov Shimcha Ulcha Na’eh Lehodot”, one may add it in right there by saying “Modim Anachnu Lach Al Ha’Nissim” etc., and continue on as usual with “Ve’al Kulam” until the end of the Amida.
However, if one remembers one’s omission only after one has mentioned Hashem’s name, one has lost his opportunity to recite Al Ha’Nissim and must continue with the usual text of the Amida. (One may not even end the blessing by saying “Lamedeni Chukecha” in order to go back and say Al Ha’Nissim.)
Inserting “Al Ha’Nissim” at the End of the Amida
Some Acharonim, including the Eliya Rabba and the Ma’amar Mordechai, write that it is preferable for one who forgot Al Ha’Nissim to insert it at the end of the Amida at the conclusion of “Elokai Netzor” before the verse of “Yihyu Leratzon.” One should recite, as follows: “Modim Anachnu Lach Al Ha’Nissim etc.” until the end of the paragraph, after which one should conclude the Amida.
“Al Ha’Nissim” in Birkat Hamazon
During Chanukah, Al Ha’Nissim is also added into Birkat Hamazon in the Blessing of Thanksgiving, as is printed in Siddurim, before “Ve’al Hakol Hashem Elokeinu Anuchnu Modim Lach.” If one forgets to add Al Ha’Nissim in the appropriate place, if one has not yet recited Hashem’s name at the end of the blessing (Baruch Ata Hashem Al Ha’aretz Ve’al Hamazon) one may go back and insert it. However, if one has already said Hashem’s name, one loses the chance to insert Al Ha’Nissim there. Even in this situation, though, it is preferable for one to recite Al Ha’Nissim in the section of the “Harachaman” toward the end of Birkat Hamazon. One should recite, as follows: “Harachaman Hu Ya’ase Imanu Nissim Venifla’ot Kemo She’asa La’avotenu Bayamim Hahem Bazeman Hazeh Bimei Matitya Ben Yochanan” etc.
Summary: During the holiday of Chanukah, “Al Ha’Nissim” is added in the Amida of all daily prayers as well as in Birkat Hamazon. If one forgets to add Al Ha’Nissim in the prescribed place, if one has not yet said Hashem’s name at the end of that specific blessing, one may go back and repeat it. However, if one has already mentioned Hashem’s name, one should add Al Ha’Nissim only at the end of the Amida or Birkat Hamazon, as explained above.