I heard an amazing midrash: "And it was evening and it was morning, one day" - from here [we know] that Rosh Hashanah is two days. And I was told that it is brought on page 48 of the recently published small book, Korban Aharon, by the sage, the honorable teacher, Rabbi Aharon Meutei, may his memory be a blessing for life in the world to come. And we cannot be exacting about this essay, for it is all one piece. But if it is true, it is possible to explain that which I saw in the just newly published book, Yaarot Devash, p. 70, that the reason that Rosh Hashanah is two days is because the sixth day - upon which the first man was created, which was Rosh Hashanah - and both the night and the day of Shabbat, were light. So it did not become dark until the conclusion of the Shabbat. This is like the Sages said in Midrash Socher Tov on Psalm 92: The night of the first Shabbat shone like the day, and there was no darkness until the conclusion of the Shabbat. And hence the two days of Rosh HaShanah are like one elongated day. For with the creation of the first man, the sixth day and Shabbat were one day. And he wrote at length about this from one matter to another, see there at length. And it is possible that this is [the meaning of] that which is written, "And it was evening and it was morning, one day." And from this [we know] that the Holy One blessed by He, gave a law about the day, that it should be evening and morning, night and day. So the first one [like this] was the first day, and all of the days [followed] after it. And since the Shabbat day that came [after] the sixth day upon which Adam was created was Rosh Hashanah - it was changed for the good and there was no darkness and light, evening and morning. Rather it was all light. From this it came out to make Rosh Hashanah, two days that are like one extended day. And from where is this known? From the boundary that the Holy One, blessed be He, set up - "And it was evening and it was morning, one day"; and likewise all [the days], that it was dark and [then] light. From this it comes out that since [that first] Shabbat was completely light, Rosh Hashanah is two days. Be precise and you will find it simple.