After all the miraculous events of Chanukah – the victory of the few and weak over the many and strong, the retaking and rededicating of the Temple, and the oil for the Menorah that lasted for eight days – there was good reason to celebrate. Subsequently, the Rabbis established Chanukah as a festival:

“And therefore the Sages of that generation enacted that these eight days, beginning with the 25th of Kislev, should be days of joy and Hallel. Every night of the eight days people kindle the lights at the entryways of houses, in order to show and reveal the miracle. These days are called Chanukah, when eulogy and fasting are forbidden, as on the days of Purim. Lighting the lights on them is a commandment from the Rabbis, like reading the Megillah” (Rambam, “The Laws of Chanukah”, 3:3).

We are obligated to obey Rabbinic decrees: According to the instruction that they shall teach you, and according to the judgment that they shall tell you, shall you do; you shall not deviate from the matter that they tell you, right or left (Devarim 17:11). (See Sefer HaChinuch, ascribed to either R. Aharon HaLevi or R. Pinchas HaLevi of Barcelona, mid-13th Century, ¶ 495, ¶ 496; Rambam, “Laws of Rebels” 1:1-2). Furthermore, the Torah authorizes the Sages to institute laws, as the Rambam says, “according to that which the time requires, namely the enactments, decrees and customs … which they order the public to obey, so as to strengthen the religion and to improve society” (ibid.).

Then again, the Torah prohibits adding to (not to mention subtracting from) it: You shall not add to the matter that I command you, nor shall you subtract from it, to keep the commandments of Hashem, your G-d, that I command you (Devarim 4:2). All the matter that I command you, shall you take care
to do: you shall not add to it, nor shall you subtract from it (13:1). (See Rambam, Book of the Commandments, Negative Commandments 313, 314; Sefer HaChinuch, ¶ 454, ¶455; Sifri 13:55; Rosh Hashana 28a; Sanhedrin 88a; Zevachim 80a; Tur and Shulchan Aruch Orach Chayim 128:27.) Consequently, if one added a fifth species on Sukkot, or if a kohen adds his own blessings to the Priestly Blessings, he is in violation of you shall not add (bal tosif).

So, by what right do the Sages enact additional laws?

Rambam was aware of this difficulty. He explains (“Laws of Rebels” 2:9) that wherever the Sages institute a law, they must make it clear that they are not adding to the Torah in any way, but only building a “fence around the Torah,” as a means of helping the public to avoid violating the Torah’s laws.

Similarly, a Rabbinic enactment is not a violation of bal tosif as long as the Rabbis declare that they do not intend to emend the Torah itself. After all, as Sefer HaChinuch (¶ 454) says, “The Master Who commands us regarding the Torah, may He be blessed, is absolutely perfect, and all His deeds and all His commands are perfect and good. Addition to them is detraction; all the more so diminishing.” Altering the mitzvot is considered an attempt to “improve” on them, which is an affront to Hashem. The Sages absolutely have no wish to “improve” on the Torah, but only to protect it and to enable its eternal values to respond to new conditions. According to Ramban, however, Chanukah poses a particular problem. On Devarim 4:2, he says that bal tosif includes inventing mitzvot, such as adding a holiday, as Yerov’am ben-Nevat, king of Israel, did (see Melachim I 12:33). In fact, GRA (R. Eliyahu ben Shlomo Zalman, 1720- 1797) in Aderet Eliyahu identifies two types of bal tosif: 1)  altering any of the mitzvot (Devarim 13:1); and 2)adding  new mitzvot (4:2). As for Purim, says Ramban, the Sageswere most hesitant to institute it until, via ruach hakodesh, they found support for it in the Torah (Megillah 14a, Jer. Megillah 1:7).

But, what is Ramban’s view regarding Chanukah, especially in view of the fact that after the days of Mordechai and Esther, there was no ruach hakodesh?!

Minchat Chinuch (R. Yoseph ben Moshe Babad of Tarnopol, 1800-1874) says that since the Torah authorized the Rabbis to institute laws, Rabbinic decrees are, by definition, not violations of bal tosif. Furthermore, the reasoning of the Sefer HaChinuch is relevant to the time when the Torah was given, “But as for that which He did not command then, and afterwards there arose the basis for the Sages’ decision to institute, such as the Chanukah light as a result of the miracles in the days of the Greeks, and the like, which were not then at the time, there is no diminishment at all in the command of the Master, may He be blessed, so bal tosif does not apply at all.” Clearly, he rejects Ramban’s position. But what would Ramban say?

Maharsha (R. Shmuel Eliezer ben Judah HaLevi Edels, 1555-1631) on Megillah 14a says regarding Chanukah, “The Sages also found in some exposition a hint in the Torah, as is found in the Midrashim.” But he does not elaborate.

An answer might be found in Ramban’s commentary on Bamidbar 8:2. He refers to the Megillat Setarim of Rabbenu Nissim of Kairouan (c. 990-1062), who quotes an obscure midrash: “When the twelve tribes brought the dedication-offerings and the tribe of Levi did not, the
Holy One, Blessed be He, said to Moshe, ‘Speak to Aharon, and say to him: When you light the lamp’ – there is another dedication in which there will be
lighting of the lamps, when I will perform through your sons miracles and salvation for Israel, and a dedication which will be called by their name,
namely, ‘the Chanukah of the sons of the Hasmonean.’” Ramban sees a similar idea elsewhere (Tanchuma Behaalotecha 5; Bamidbar Rabba 15:5). (Thank you to Rav Nechemia Reichman for suggesting this solution.)

According to Ramban, there is no violation of bal tosif, since, like Purim, the institution of Chanukah and its lights are predicted in the Torah.