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.