The Mishna in Succah describes in detail the Simchas Bais HaSho'eva that took place in the Bais HaMikdash on Succos. The question is what were they celebrating? There is a special mitzva on Yom Tov to be happy, "V'Samachta B'Chagecha". There is also a special mitzva on Succos of "Nisuch HaMayim"; to pour water on the mizbay'ach with the korbonos in addition to the wine that was poured all year round.
Rashi in a number of places says that Simchas Bais HaSho'eva was in
honor of the pouring of the water on the mizbay'ach and was only a
Mitzva D'Rabanan. Similarly today it is only a zecher for the the
Simchas Bais HaSho'eva that took place in the times of the Bais
HaMikdash.
The Rambam says (Lulav 8:12-15) that festivities of Simchas Bais HaSho'eva are part of the mitzva of being happy on Yom Tov. He says that instruments were brought to the Bais Hamikdash and there was great singing and dancing. He makes no mention whatsoever about the pouring of the water on the Mizbay'ach. This means that Simchas Bais HaSho'eva is a Mitzva D'Oiraisa, not only in the times of the Bais HaMikdash but even today.
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