I don't do Sundays, haven't got the patience. I also find if you set a close time on Saturdays, say (2.0pm) , most people who really are serious will book an appointment in advance, so these are the ones to concentrate on.
I find most customers on Sundays are just at a lost of what to do for the day, so go and test a few cars, usually bringing the whole family …
I am sure I miss a few sales, but it seems to work ….