On Chanukah and Purim, we say the blessing, Asher Kidishanu - "Which God has commanded us..." Yet aren't those words inappropriate, given that nowhere in the 613 mitzvot of the Torah are these holidays mentioned.
The Aish Rabbi Replies:
There are two types of mitzvahs: Torah-level and Rabbinic-level.
Torah-level refer to the 613 mitzvahs delineated in the Five Books of Moses.
Rabbinic mitzvahs are legislated by the Sages. The reason we say the same blessing ("which God has commanded us") is because the Torah itself gave the power of legislation to the Sages, as it is written, "You shall not stray from the word that [the Sages] tell you, whether to the right or to the left" (Deut. 17:11). Therefore, since the Torah gave the power to the Sages to legislate mitzvahs, it was as if the Torah itself sanctions these mitzvahs.
Rabbi Moshe Chaim Luzzatto wrote: "God's authority decreed that these Rabbinic mitzvahs be kept just like the Torah mitzvahs... One who violates these transgresses God's word as much as one who violates the explicit commandments."
There are seven Rabbinic mitzvahs that have this blessing. They are:
1) washing of the hands before bread
2) lighting of Sabbath Candles
3) lighting of the Festival candles
4) making an Eruv (carrying area for Shabbat)
5) reciting Hallel (Psalms of praise)
6) lighting the Chanukah menorah
7) reading the Megillah on Purim