JavaScript, PHP and various other web-based languages have date functions whose weeks begin on Sunday and end on Saturday. That is, as individual days, Sunday is identified 0 and Saturday is 6 (with Monday to Friday in between). Logically then, a calendar should start on Sunday and end on Saturday

However, a lot of calendars begin their weeks on Monday. Google's Calendar being one of them.

Does it matter at all from a user experience perspective what day I start my calendar on?
Would users expect to be able to switch between Sunday and Monday as the start of a week?