How and why does a Jubilee year cause Rosh Hashanah to take place on the 10th day of the month?

Question

How and why does a Jubilee year cause Rosh Hashanah to take place on the 10th day of the month?

Ezekiel 40:1 reads "on Rosh Hashanah, on the tenth of the month". Rashi asks and answers, "'What “year” is it whose Rosh Hashanah falls on the tenth of the month? The answer is that this is the Jubilee year.'" From whom (or from what work) did Rashi source his answer?

How is it that the (or a) Jubilee year causes the following: "Rosh Hashanah falls on the tenth of the month"?

Note: I only have one question (as stated above), but I would like to know if the answer also answers the following: A. Is there a rabbinic opinion that the "Rosh HaShanah" mentioned by the Prophet Ezekiel refers to the first of Nissan? B. Is it ever possible that Rosh HaShanah again falls on the 10th of the month? C. Did Rosh HaShanah always fall on the 10th of the month during a Jubilee year?

Answer 1

IIRC, the explanation goes like this:

1. There's no Biblical festival of R"H, there's one of blowing the Shofar on the first day of the 7th month.

2. On Jubilees, the main Shofar (to free the slaves, etc) is blown on the 10th day of the 7th month as it says:

"וְסָפַרְתָּ לְךָ שֶׁבַע שַׁבְּתֹת שָׁנִים שֶׁבַע שָׁנִים שֶׁבַע פְּעָמִים וְהָיוּ לְךָ יְמֵי שֶׁבַע שַׁבְּתֹת הַשָּׁנִים תֵּשַׁע וְאַרְבָּעִים שָׁנָה׃
וְהַעֲבַרְתָּ שׁוֹפַר תְּרוּעָה בַּחֹדֶשׁ הַשְּׁבִעִי בֶּעָשׂוֹר לַחֹדֶשׁ בְּיוֹם הַכִּפֻּרִים תַּעֲבִירוּ שׁוֹפָר בְּכָל־אַרְצְכֶם׃
וְקִדַּשְׁתֶּם אֵת שְׁנַת הַחֲמִשִּׁים שָׁנָה וּקְרָאתֶם דְּרוֹר בָּאָרֶץ לְכָל־יֹשְׁבֶיהָ יוֹבֵל הִוא תִּהְיֶה לָכֶם וְשַׁבְתֶּם אִישׁ אֶל־אֲחֻזָּתוֹ וְאִישׁ אֶל־מִשְׁפַּחְתּוֹ תָּשֻׁבוּ׃

"You shall count off seven weeks of years—seven times seven years—so that the period of seven weeks of years gives you a total of forty-nine years. Then you shall sound the horn loud; in the seventh month, on the tenth day of the month—the Day of Atonement you shall have the horn sounded throughout your land and you shall hallow the fiftieth year."

This is what Rashi explains on Erchin 12a:

"איזו היא שנה שראש השנה בעשור לחדש הוי אומר זה יובל"

Rashi: "ראש השנה שלה ביום הכפורים דתוקעין ב"ד בשופר ונפטרים עבדים לבתיהן ושדות חוזרות לבעליהן ואז נראית שהיא יובל"

As we can see, the blowing (aka Rosh Hashana) takes place on Yom Kippur, exactly as Rashi interprets.

Answer 2

According to Menachem Zohari in his series of books on the sources of Rashi's commentary on Tanach, "Mekorot Rashi", in the book on Yechezkel, Rashi's sources for this commentary are:

1. The Talmud in Erchin 12a, as brought in the above answer by @AlBerko.

2. Two sections of Seder Olam Rabbah:

a. Chapter 11 (my translation):

"...and so it says "In the twenty-fifth year of our exile on Rosh Hashanah etc", when was this said to him? In the beginning of the Jubilee..."

b. Chapter 24 (my translation):

"Yoshiyahu was eight years old when he began to rule and he ruled for thirty-two years in Yerushalayim etc (Kings 2:22) in the eighteenth year, on that year a Sefer Torah was found in the House of the LORD and that year was the beginning of the Jubilee..."