Do they clean
the car every Sunday? - Yes, they do.
Do they clean the car every Sunday or every Saturday? – every Sunday
Who cleans the car every Sunday? - they
They clean the car every Sunday, don't they? - Yes, they do.
What do they clean every Sunday? - the car
How often do they clean the car? - every Sunday
When do they clean the car? - on Sundays
What do they do every Sunday? – they clean the car