While most countries set Monday as the first day of the week, some may set Sunday or Saturday as the first day. To change start of week on iPhone calendar, do as follows:
1. Open the Settings from the home screen
2. Tap to open to the Mail, Contacts, Calendars settings from the list of default apps of your iPhone
3. Scroll all the way down to the Calendar settings and tap to open the Start Week on option
4. Now select the day as the first day of the week.