at (time): 時の一点、何時、何分、正午、真夜中など
on (day): 特定の日、何曜日、何日の午前/午後など
in (season/month/year): 比較的長い期間、月、四季、年など

at 9:00, at one oclock, at dawn, at sunset, at noon, at midnight, at Christmas
on Saturday, on May 6, on the weekend, on Christmas day
in April, in 2007, in April 2019, in the 21st century

日付まで on、月だけ in
on March 2
in March

1日はon : on my birthday/Friday/Christmas day/May 25等
2日以上はin : in the month/the past week/winter等

詳しく
曜日・日付・時間・場所でバリエーション
at six in the morning of 20 April
at 4:00 pm on Friday, 12 February 2010
on Wednesday, August 13, 2:00-3:00
on Wednesday, Jan. 18, at the Moultonborough Library at 7 p.m.
at 5:00 p.m. on Saturday, December 10, 2016
at 5.00 p.m. on Saturday 10 December 2016
Saturday, April 28, 2018
Friday, May 4 2018, 4:00 pm, Yoyogi Park
on Friday, May 5 2018, at 9:00 am in the conference room
https://bebeblanchecoco.com/date-time-location/
書く：一般的には「時刻、曜日、月、日、年」または「曜日、月、日、年、時刻」
March 28, 2012 at 07:03 PM EDT
29 March 2012 at 10:40
March 28, 2012, 11:28 p.m. ET
Thursday 29 March 2012 06.43 EDT
10:57AM BST 29 Mar 2012
https://english-columns.weblio.jp/?p=439

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in the morning
なぜin the morningなのか、広がりのある時間帯をさすことばだから[中に]のinを取る

特別な日、修飾する語が付くと限定的になりinからonに（新聞等ではonが省略されることもある）
on Monday morning
on the afternoon he has born
on the morning of April 28
on the morning of his departure
on a cold morning

しかしこの場合はinのまま（主体が朝だから）
in the fresh of the morning
early in the morning

しかしこれはon（主体が朝でも限定されているから）
by 10 am on the morning of February 25

日時にthisやevery等が付く場合は副詞なので前置詞は不要
I go to church every Sunday.
I will go to church this morning / tomorrow morning /the following morning.

読みは序数でありfirst, second, thirdだが、十の位はeleventh等なのでth
11th, 12th, 13th
21st、22nd、23rd
31st

12時の読みには0ゼロを使わない
twelve noon, twelve midnight, 12AM, 12PM