On the morning of October 20
On the morning of the wedding
這用法的表達重點是在那個特定日期（on … October 20），當天的那個 “the morning” 倒是其次。我們有時記得的是一個特定事件「那天」，這時是用那個事件代表那天，這表達習慣中文和英文一樣。
1. https://www.ldoceonline.com/dictionary/morning
找 GRAMMAR: Patterns with morning
...
on the morning of something
• Something happens on the morning of a particular date or event:
- On the morning of March 15,2008, he was involved in a car accident.
- Make sure you arrive in plenty of time on the morning of the exam.
2. http://www.merriam-webster.com/dictionary/morning
例句: She arrived on the morning of March 18.
3. https://dictionary.cambridge.org/us/grammar/british-grammar/at-on-and-in-time
找 In or on? "... The ship left the harbour on the morning of the ninth of November."
4. https://www.oxfordlearnersdictionaries.com/us/definition/english/morning_1
- on the morning of something We got the news on the morning of the wedding.
但是 …
和 late 或是 early 搭配時的慣用表達則是用 in。比如 late in the morning of June 19, 1967。
這用法表達重點除了日期之外也特別將注意力轉移到特定的區段（以上例為例，不是 early in the morning 喔，是 late in the morning）。
1. https://www.ldoceonline.com/dictionary/in-the-morning
Examples from the Corpus:
- Late in the morning of June 19,1967, he received a call from Atty.
2. English in Mind Level 2B Combo, by Herbert Puchta, Jeff Stranks, Cambridge University Press, page 12
- ... it happened very early in the morning of April the eighteenth, 1906.
後記
說老實話，我第一次在網路上碰到有人問這問題時我也以為是 in。我當時不知道用 on 的這用法，因為之前沒碰過——不管是日常，在讀研究所時或是後來上班，一般要嘛講個大概日期，要嘛如同商業書信那樣明確的寫好時間和日期，沒必要在那邊另外加個模糊性質的 the morning。不過在那之後我在新聞媒體上碰到時我倒是有特別注意這用法。
編按：以下為舊資料
