Plugin wont work for just Sunday as delivery day

Hi guys, first off this plugin is absolutely brilliant! such a great user interface and so easy to use.

There is just one small bug i have found with it. Our client just wants to have the only delivery date as sunday (as its for delivering sunday brunch). When i check the box for just sunday the option doesn’t save. however if i select one more day eg. saturday the options save and the calendar greys out the days accordingly?

Would you guys be able to tell me why sunday on its own doesnt save? is there anyway we could change this as i would love to stay using this plugin.

Thanks a mil, Ed