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This question focuses on the term "Matan Torah" used everywhere, its practical and factual events (no metaphors, please). Also, this is not about the festival of Shavuos as in this question.
I've heard a lot of metaphorical descriptions of Maamad Har Sinai, like the revelation of G-d Himself, the visions, the voices, the feeling of closeness and oneness. After all the "bells and whistles", if I get it right, what actually happened at Mt Sinai, as Torah itself describes it, is that the Jews had heard/seen the first 2 commandments from G-d and the rest 8 from Moses.
Then the big show was over, everyone returned to their tents and Moses went up to study for 40 days and receive the Luchos in writing. Forty days later the Golden Calf incident happened. Then another forty days of Moses's seclusion, and then he came back on Yom Kippur.
To sum it up - there were the Decalogue and the Luchos, but what I don't see anywhere in this happenings is Matan Torah - the giving of the Torah as I know it - the 5 Books of Moses.
Moreover, the Torah (or NAC"H) doesn't have such a term or a day to celebrate Matan Torah at all, so it is clearly was made much later. Even if we follow the approach that Moses received a full finished Torah scroll after he descended the second time, Yom Kippur should be called Matan Torah.
So: Why the Sinai revelation of the Decalogue (and/on the Luchos) is called "Matan Torah" and who established it?