When the first night of Chanukah falls on Friday night, many people have the practice of reciting Mincha early on Friday afternoon, before candle lighting. Most communities therefore conduct an earlier Mincha service on Friday afternoon in such cases, and people then light the Chanukah candles before Shabbat and come to the synagogue for Arvit.
The Sha'ar Ha'tziyun (supplementary notes to the Mishna Berura by Rabbi Yisrael Meir Kagan, the "Chafetz Chayim," Lithuania, 1839-1933), in Siman 679, suggests a possible reason for this custom. Namely, once an individual lights the Chanukah candles, he establishes that the day of Erev Shabbat and Erev Chanukah has ended, and the period of nighttime, which ushers in the first day of Chanukah, has begun. If, indeed, night has fallen as far as this individual is concerned, then he can no longer pray Mincha, the afternoon prayer; reciting a daytime prayer after one has already established that nighttime has begun would constitute a "Tartei De'satrei," a Halachic paradox. The custom therefore evolved to recite Mincha earlier in the day, before candle lighting.
Although it is preferable to follow this practice, nevertheless, if a person did not recite Mincha before he lit Chanukah candles in this case, he may still recite Mincha, so long as the sun has not set. The question, however, arises as to whether he should add the Al Ha'nissim insertion in the Amida during Mincha. Since, as mentioned, he has determined by virtue of his lighting that night has already fallen, and Chanukah has thus begun, perhaps he should be required to insert Al Ha'nissim in his Amida. The problem, of course, is that inserting Al Ha'nissim would merely magnify the contradiction inherent in this individual's recitation of Mincha: Al Ha'nissim can be recited only if Chanukah has begun, and if Chanukah has indeed begun, then one can no longer recite Mincha, which is relevant to the afternoon before Chanukah.
The widespread practice is to recite Mincha in such a case without adding Al Ha'nissim. The explanation for this practice perhaps relates to the dual nature of the Chanukah celebration. We observe this festival in commemoration of two miracles: the Chashmonaim's stunning military victory over the Greeks, and the miracle of the jug of oil. We commemorate the miracle of the oil by lighting candles each night of Chanukah, and we celebrate the military victory through the recitation of Hallel and Al Ha'nissim. Therefore, when one lights the Chanukah candles on Friday afternoon before the first night of Chanukah, although he has indeed ushered in one aspect of Chanukah – the celebration of the miracle of the oil - he has not established the onset of the second aspect of this festival, the commemoration of the military triumph. Hence, although one has already lit the Chanukah candles, he may still recite Mincha without inserting Al Ha'nissim, as these two observances – candle lighting and reciting Al Ha'nissim – relate to two distinct aspects of the holiday. Preferably, however, one should recite Mincha before lighting the Chanukah candles when the first night of Chanukah falls on Friday night.
If one did add Al Ha'nissim during Mincha in such a case, he need not repeat the Amida. For one thing, the Shulchan Aruch rules (108) that if one mistakenly added an unnecessary insertion to the Amida, such as one who adds "Ata Chonantanu" (which is inserted in the Amida on Saturday night) during the week, the Amida is still valid. Furthermore, as noted by Rabbi Moshe Feinstein (Russia-New York, 1895-1986) in his work Iggerot Moshe, Al Ha'nissim is not entirely out of place on Erev Chanukah, as miracles occurred for the Chashmona'im already on the 24th of Kislev, during their battles against the Greeks. Hence, although one should not add Al Ha'nissim during Mincha of Erev Shabbat in such a case, one who did would not be required to repeat the Amida. This is the ruling as well of Chacham Ovadia Yosef, in his work Chazon Ovadia (Laws of Chanukah, p. 188).
A similar question arises when one begins a meal in the afternoon of the final day of Chanukah, and the meal extends into the nighttime hours. Does he recite Al Ha'nissim in Birkat Ha'mazon, since the meal began on Chanukah, or does he omit Al Ha'nissim given that Chanukah has already ended?
The Shulchan Aruch explicitly rules (695:3) that in such a case one would, in fact, add Al Ha'nissim to his recitation of Birkat Ha'mazon. Just as one whose Se'uda Shelishit (third meal) on Shabbat extends until the nighttime hours nevertheless adds Retzei in Birkat Ha'mazon, so does a person add Al Ha'nissim to Birkat Ha'mazon if he began his meal on Chanukah, even if it ended after dark. This is, indeed, the ruling of Chacham Ovadia Yosef (Chazon Ovadia, Laws of Chanukah, p. 207).
Summary: When the first night of Chanukah falls on Friday night, one should preferably recite Mincha before lighting the Chanukah candles. If he lit the Chanukah candles without first reciting Mincha, he may nevertheless recite Mincha after candle lighting, and he does not add Al Ha'nissim in the Amida of Mincha; if he did add Al Ha'nissim to the Amida in such a case, he does not repeat the Amida. If one began a meal on the eighth day of Chanukah and it extended into the nighttime hours, he adds Al Ha'nissim during Birkat Ha'mazon.
