How to Log Workouts
You can log a workout from the homescreen by tapping the top card marked NEXT or the + button in the top right corner.
The date of new workouts is today by default. But you can change this by tapping the date in the navigation bar (top right when you open the workout). If you already logged a workout today, the date will show as tomorrow. You can change this but don't need to since the date automatically updates on a daily basis.
You can also log workouts from the calendar by tapping a date without red circle. This automatically adds a workout on that day.
If you skip a workout, don't log it. You don't need to log a workout with blank reps or anything similar. Just don't the workout in the first place.
Your calendar will show no workout for that particular day. And the app will properly change the weight if it takes you longer than a week to get back to training.
For more information on what to do if you skip a workout, see this: