if moshe rabeynu made an extra day of prisha from his own mind how can u learn the din of "shichvat zera masrachat" after 3 days?
The Gemara says that Moshe Rabeinu derived ("Darash") this from the verse using a Hekesh (see there, 87a). When the Gemara says "m'Da'ato" -- "on his own initiative," it means merely that this extra day was not written explicitly in the verses and Moshe derived it by using the 13 hermeneutical rules with which laws are derived from the Torah. (This, by the way, is a support for the words of the Rambam, who always uses the term "Divrei Sofrim" and not "Divrei Torah" to refer to laws that were derived through the 13 rules.)
Kol Tuv!
-Mordecai
When the Rambam refers to divrei kabalah as opposed to divrei soferim, is this intended to be the Rambam's lashon to differentiate dinei drabanan from dinim learned from navi?
The Rambam uses the term "Divrei Sofrim" or "Divrei Kabalah" to refer to one of two categories of Halachah:
[1]
Verses from the Navi (as opposed to verses in the Torah). The Mishnah (Ta'anis 15a) also uses this term to refer to verses in Navi (see Rashi there who explains when we call the words of the Navi "Divrei Kabalah"). We find this use of Divrei Kabalah in the Rambam in a number of places (see, for example, Hilchos Ta'anis 4:7, Isurei Bi'ah 12:6, Talmud Torah 1:14).
[2]
(a) Halachos that are learned from the 13 Midos (Hilchos Isurei Bi'ah 4:14, Ma'asei ha'Korbanos 14:2, Parah Adumah 7:1, etc.).
At times, the Rambam cites a source "mi'Pi ha'Shemu'ah" for the Divrei Kabalah (Hilchos Kela'im 9:12, Tum'as Ochlim 12:1, Kelim 3:1). Other times, the Rambam tells us about "Divrei Kabalah" that they are learned "m'Pi Moshe Rabeinu b'Sinai" (Hilchos Mechusrei Kaparah 2:10).
(b) The Rambam makes it clear that these Halachos are not in the same category as Halachos that are written explicitly in the Torah (as the Rambam writes in Hilchos Tzara'as 15:1, Tum'as Meis 5:7); rather, they are a type of Divrei Sofrim (see Tum'as Meis 19:18). (The Rambam is consistent with his opinion that even a Halachah l'Moshe m'Sinai is considered "Divrei Sofrim," because it was the Sofrim, the Sages, who passed along the Halachah and it was not written explicitly in the Torah.)
(c) As to why the Rambam calls these Divrei Kabalah and not Divrei Sofrim, the Rambam writes in Hilchos Mamrim (1:3) that "There are no disputes whatsoever about any Divrei Kabalah. If a dispute is found regarding a Halachah, it is certain that that Halachah is not [passed down directly] from Moshe Rabeinu" (See also what the Rambam writes in his introduction to Perush ha'Mishnayos). It is clear from these words that the Rambam is using the expression Divrei Kabalah to refer to laws which are Halachah l'Moshe m'Sinai. Although he brings sources from verses for some of the Divrei Kabalah (as cited above), they may still be Halachah l'Moshe m'Sinai. The Rambam writes in his introduction to Perush ha'Mishnayos that there are Halachos which are Halachos l'Moshe m'Sinai for which the Sages suggested verses as support, but nonetheless they are not derived from the verses but are Halachos l'Moshe m'Sinai.
Thus, wherever the Rambam writes that a Halachah is Divrei Kabalah, it is apparently a Halachah l'Moshe m'Sinai.