Ashita and Asu and Myonichi

The Japanese word "明日," which means "tomorrow," has three different readings, "ashita," "asu," and "myonichi."

In general, Japanese people often use either "ashita" or "asu."

"Ashita" originally meant "朝 (asa)," which means "morning," and used as an antonym of "夕べ (yube)," which means "evening."

Then the meaning came to change to "next morning," "next whole morning," and "next day."

On the other hand, "asu" means "tomorrow" from the beginning, and it seems to come from the abbreviation of "akasu," which means "dawn," or "asa," which means "morning."

Also, "asu" sometimes means "in the near future."

"Myonichi" is the just reading of "明日" in on-yomi (Chinese reading).

The most formal reading is "myonichi," the second place is "asu," and the most casual reading is "ashita."

「あした」と「あす」と「みょうにち」

日本語の「明日」という漢字は、「あした」「あす」「みょうにち」の三通りの読み方があります。

世間一般では、「あした」もしくは「あす」のどちらかが使われることが多いです。

「あした」はもともと「朝（あさ）」を意味し、「夕べ（ゆうべ）」の対語として使われていました。

そして、夕べに対する次の朝、次の午前中、次の日（明日）という意味に変化していったそうです。

一方、「あす」は「明かす（あかす）」の略、または「あさ（朝）」から転じたものと考えられており、昔から「翌日」を意味していました。

「あす」は、「近い将来」を表すこともあります。

「みょうにち」は、「明」と「日」をそのまま音読みしたものになります。

最もフォーマルで文語的なのは「みょうにち」、次いで「あす」、そして最もカジュアルで口語的なのは「あした」です。