The built-in Calendar app on your iPhone and iPad has a few extra add-on features that you can enable if you choose. Week numbers is one of them and lets you easily view what week number of the year it currently is and what week any date falls on, which is much easier than counting weeks manually!
How to turn on week numbers in Calendar for iPhone and iPad
- Launch the Settings app on your iPhone or iPad.
- Tap Mail, Contacts, Calendars.
- Turn On the option for Week Numbers, which can be found under the Calendars subsection.