For an individual user to change the first day of the week to Sunday on Team Calendar, it is necessary to choose the English US as a language on (Browse > User Settings > Language), but the English US language is not bundled in Confluence, and in OnDemand instances is not possible to download any language packages. See TEAMCAL-121 for more information on this.
Also, per OnDemand restricted functions:
Alternative Languages —There's no full support for alternative languages except English (US) at the moment. Please follow JST-801 for updates on this feature.