the calendar takes it's start/end days of the week from the local language pack.
if you want it to start on a Sunday, you will need to install the US pack and change your local user settings to US English. You will have to do this for every user so it's not a solution for a large install.
It's a common request, if you look at their Jira you will find:
Atlassian Summit is an excellent opportunity for in-person support, training, and networking.Learn more
Hey everyone! My name is Natalie and I'm an editor of the Atlassian Blog and I've got a question for you: What's your favorite quote about teamwork? We've compiled a list here, along with...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
You're one step closer to meeting fellow Atlassian users at your local meet up. Learn more about AUGs