Switch Week Numbers in IOS Calendars With This Setting
If you’re not very picky about your calendar settings, you probably never went to the iOS settings menu for your calendar. But OS X Daily points to one handy toggle switch you may never have noticed: week numbers.
Just go to Settings> Mail, Contacts> Calendars and turn on the Week numbers option. Now you will see the week number in the calendar app, which is very handy if you work in a profession that needs it.
How to show the week number on the calendar for iPhone and iPad | OS X Daily