How to Use The Calendar (iPhone)
The calendar in the app gives an overview of all the workouts you've done so far.
By default it's empty because you haven't done any workouts. Each time you log one, the date will show inside a red circle. Tap this red circle to view the workout and maybe edit it. Your ongoing workout will show in a lighter red circle.
The calendar is not a planning tool for future workouts. It won't show when to workout in the coming weeks. The home screen will show you this - your next 3 workouts show on the homescreen. You can set your training schedule by tapping homescreen - edit - set the workout days here.
If you do StrongLifts 5x5, the recommendation is to train 3x/week - most people do Monday/Wednesday/Friday but any similar schedule works. Read https://stronglifts.com/5x5/
Use the calendar to review what you've done so far. Aim to get three red circles every week. Go back to a particular workout by tapping the matching date with the red circle.
Your phone settings determine the first day of the week in the calendar. If your phone language is set to English (US), the calendar starts on Sunday because that's the standard in the USA. If it's English (UK), the week will start on Monday like in the UK. Change your phone settings to change the first day of the week.