InなのかOnなのか。

正しい文章は In July 2021, those islands were registered as World Natural Heritage sites.

月や年の場合はin
日付や曜日の場合はon

Christmas is in December.
On December 25, we opened our Christmas presents.
Christmas falls on Saturday this year.
In the 1990s, Japan's bubble economy crashed.
On October 1, 2019, Japan's consumption tax increased to 10%.
He swam on Wednesday.