There are 12 names, themes and idioms for Rosh Hashanah. Messianic Rabbi Eddie Chumney has written a very thorough teaching on Rosh Hashanah. I realize someone else just posted all this material, I am putting up only one part. Here is what Rabbi Chumney has written regarding Yom Hakeseh, another name for Rosh Hashanah, “the hidden day”, the day no man knows.
Yom HaKeseh: The Hidden Day
In Psalm (Tehillim) 27:5 it is written, "For in the time of trouble He shall hide me in His pavilion; in the secret of His tabernacle shall He hide me; He shall set me up upon a rock."
Yet another name for Rosh HaShanah is Yom HaKeseh, "The Day of the Hiding" or "the Hidden Day." The term keseh or keceh is derived from the Hebrew root kacah, which means to "conceal, cover, or hide." Every day during the month of Elul, a trumpet is blown to warn the people to turn back to G-d, except for the thirtieth day of Elul, the day preceding Rosh HaShanah. On that day the trumpet is not blown, and is therefore silent. This is because much about Rosh HaShanah is concealed and shrouded in mystery. The mystical aspect of Rosh HaShanah is indicated in Scripture: "Sound the shofar on the New Moon, in concealment of the day of our festival" (Psalm [Tehillim] 81:3). Satan, the accuser, is not to be given notice about the arrival of Rosh HaShanah, the Day of Judgment.
Rosh HaShanah is called Yom HaKeseh, or the Day of the Hiding, because it was hidden from satan (Ha satan), the adversary. The Bible says that satan comes to rob and to steal (John [Yochanan] 10:10, and to confuse (1 Corinthians 14:33). Because it is the Day of Judgment, it is symbolically hidden from satan (satan did not know and understand the plan of the cross [tree], First Corinthians 2:7-8). This was hidden from him as well. Believers never said when the day of Rosh HaShanah was; they simply said, "Of that day and hour no one knows, only the Father."
One of the reasons most often given to disclaim that the resurrection of the dead and the catching away of the believers is on Rosh HaShanah is the statement given by Yeshua in Matthew (Mattityahu) 24:36, as it is written, "But of that day and hour knoweth no man, no, not the angels of heaven, but My Father only." Because Rosh HaShanah was understood to be the hidden day, this statement by Yeshua is actually an idiom for Rosh Hashanah. Thus it should be given as proof that He was speaking of Rosh HaShanah because Rosh HaShanah is the only day in the whole year that was referred to as the hidden day or the day that no man knew.
Spiritual Application (Halacha). Rosh HaShanah takes place on the new moon. Colossians 2:16-17 says that the new moon will teach about the Messiah. The Jewish (biblical) month is based upon a lunar cycle. The moon can barely be seen as the cycle begins. But then the moon turns toward the sun and begins to reflect the light of the sun. The sun in the sky is a picture of Yeshua (Malachi [Malachie] 4:2), and the moon is a picture of the believers in the Messiah. The sun has its own light, but the moon's light is a reflection of the sun. When we first become believers in Yeshua, we can hardly be seen spiritually, and we know very little about G-d. But then our lives begin to revolve around the Messiah as the moon revolves around the sun. As we begin to turn more and more toward the center of creation, we begin to reflect that light (Yeshua) more and more, just as the moon reflects the light from the center of the solar system.