In my writing, I usually start the words for days of the week or months with capital letters.
ex. Saturday, September...... etc.,
In other people's writing, I often see these words starting with small letters.
Is starting these words with small letters acceptable?
In my dictionaries, I can't find any of them starting with capital letters.

Thank you very much for helping me in advance.
AnonymousWhich one is a proper noun?

capital mackinac ???????
she or

jane as a person's name Jane; as a slang word for a woman jane

Names should always be capitalized.
Names of days and months are proper nouns. It is only right to start a proper noun with a capital letter. One think I have learned is that we must be careful in choosing who/what to refer to. Not all dictionaries are carefully edited. And sometimes, even an English teacher can make mistake. I understand, though, it is always frustrating when what you believe as being correct is the opposite of what the whole world think.

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which one is a proper noun

capital mackinac she or jane
 Feebs11's reply was promoted to an answer.