Hi @Finn Togo Welcome to the Atlassian Community!
The Team Calendars follows per user language preferences to decide which day is at the start of the week. For example, if you want to make the start of the week to Sunday (US Calendar), you would need to install the US Language pack and edit your Confluence user preferences to choose US English. To do this, click User name > Settings > Edit > Language and then choose US English.
The Confluence default is to Automatically detect browser settings.