Setting the first day of the week for the calendar
If your browser locale is set to en-US, calendars in Trello will show Sunday as the first day of the week.
If your browser locale is set to anything other than en-US, calendars in Trello will show Monday as the first day of the week. Further, dates will be presented in a more internationally-appropriate format.
This site will show you the locale that your browser is currently requesting: http://www.localeplanet.com/support/browser.html.
In some instances, it's possible for you to have your browser set to a non en-US language, but the first day of the week still shows up as Sunday in Trello. To work around this issue, change your language in Trello, first to German, and then back to English. When you do this, your locale in Trello will "reset" and should match what you have in your browser.