Hello,
I wonder which one is correct:
in the morning of the 14th of July or
on the morning of the 14th of July?
Thank you.
"on" is the correct preposition in your context.
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You can also read here https://skyeng.ru/articles/kak-razobratsya-v-predlogah-vremeni-i-mesta-v-anglijskom the article is about prepositions)