I have asked this question to many Rabbanim, and still have not been able to get a reasonable answer, so I thought I would try asking it here, and see what people thought.

It says in Devarim 4;2:

לֹא תֹסִפוּ עַל הַדָּבָר אֲשֶׁר אָנֹכִי מְצַוֶּה אֶתְכֶם וְלֹא תִגְרְעוּ מִמֶּנּוּ לִשְׁמֹר אֶת מִצְוֹת יְ־הֹוָ־ה אֱלֹהֵיכֶם אֲשֶׁר אָנֹכִי מְצַוֶּה אֶתְכֶם

Translated:

Do not add to the word which I command you, nor diminish from it, to observe the commandments of the Lord your God which I command you.

Now I understand that the rabbis have the ability to make halachot that are either gaurds on another halacha (e.g. the concept of muktza) or interpretations of a halacha in torah (e.g. Not using electricity on shabbat, being that their was obviously no such thing as electricity when the torah was given). However Channuka and Purrim are not guards on mitzvot, nor are they interpretations of what a mitzva means. They are entirely new mitzvot! So why don't Purim and Channuka fall into the catigory of לֹא תֹסִפוּ?

At this point, I am usually given the answer: "Purim and Channuka are mitzvot derrabannun (from the rabbis) and since they don't claim that it is a mitzva deorita (a mitzva directly from torah) they were allowed to establish it".

Now the problem I have with the above statement is as follows:

There is a Gemara in brachot (27b) that says (and I'm paraphrasing) that a person who quotes his rabbi on something that he never said is causing the Shechinah to depart from the Jewish people.

Now I quote the actual bracha that is said on Channuka (it's pretty much the same for Purim, but I couldn't find a text to copy, and I don't know how to type manually in Hebrew)

ברוך אתה ה' א‑לוהינו מלך העולם, אשר קדשנו במצותיו וצונו להדליק נר של חנוכה.‏

Translation:

"Blessed are You, Lord, our G-d, King of the universe, Who has sanctified us with His commandments and commanded us to kindle the Hanukkah lights.

Now according to the bracha, it is saying that G-d himself commanded us to light the channuka candles, which is in fact not true, as the torah was given far before the story of Channuka.

Around this time, I usually get the answer: "It says in the torah that you have to listen to the rabbis, therefor, since the torah says listen to the rabbis, and the rabbis say light candles on Channuka, than the torah is saying light candles on Channuka".

Now I have a huge problem with this, because they could have very easily worded the bracha such that it said "Blessed are You, Lord, our G-d, King of the universe, Who has sanctified us with His commandments and commanded us to follow the ways of the rabbis, who have told us to kindle the Hanukkah lights."

Now I go back to the gemara that I quoted earlier. If it considered such a horrible thing to misquote your rabbi, thank Kal Vachomer wouldn't it be even worse to misquote G-d himself???

• Since most of the people here probably originated on StackOverflow, I though I would try giving an analogy to programming. Let's say I take a programming class with teacher-X. Teacher-X learned how to program directly from Mark Zuckerberg himself. On the first day of class, Mark Zuckerberg comes in, and tells us that he had personally taught Teacher-X himself, and he really knows what he is talking about, so we should listen to everything he says. Teacher-X teaches me to program using a specific style which I go on to use when I code. One day, I am writing a program with my friend, and he notices the strange style I am coding with, so he asks why I am doing it that way. I tell him, Mark Zuckerberg taught me to do it. Well, in fact, Mark Zuckerberg did not teach me to do it that way, and saying that he did was is a false statement. I learned it from Teacher-X, Zuckerberg just told me to listen to him because he had trained him. Similarly, when saying the bracha on channuka candles, saying that G-d himself commanded me to light the channuka candles is a false statement. He simply told me to listen to someone who did. And I am now quoting G-d with something he never said. And while it may not in practice be as horrible as what is said in the gemara, I would imagine it is still not a good thing to do.

So how were the rabbanim able to create Channukah and Purim in the way they did? It seems like they directly violated the laws of the torah.