Week starts on Monday or Sunday

The beginning of the week is an individual setting. And it depends on the user's global locale.

To change your locale, go to the page
https://id.atlassian.com/manage-profile/account-preferences?ref=jira&src=personalsettings

And choose the language that suits you. For example:

English US - week starts on Sunday
English UK - week starts on Monday

By the way! Count of Weeks will be different for weeks starting on Sunday from weeks starting on Monday, as the standard is different.