Unless you’re very particular about your calendar settings, you’ve probably never ventured into the iOS settings menu for your calendar. But OS X Daily points out one handy toggle you may have never noticed: week numbers.
Just hop into Settings > Mail, Contacts > Calendars and toggle the Week Numbers option to on. Now, you’ll see the week number in the calendar app, which is pretty handy if you’re in a profession that needs that.
How to Show Week Number in Calendar for iPhone & iPad | OS X Daily