Where is a diaeresis, or dieresis, used?

Question

What name is given to a special day or happening, used variously over centuries to mark important days in a calendar, to mark capitals or rubrics in religious texts, or to wear particular clothing?

Answer 1

In written English
It is written as two dots placed over what it affects, and used to indicate that one of the vowels in a diphthong (or one of two adjacent vowels in separate syllables) is pronounced separately.