Depends on the sort of thing you are selling.
Kiddies toys weekend daytime, kiddies clothes, Mon - Fri daytime.
PC games , consoles, gadgets - late nights, especially Friday and Saturday nights.
Specialist / rare / collectors' items - any time at all.
Ladies' clothing - daytime. Unless they are large enough to fit men - then it's late night shopping. :)
If there is something big on tv - avoid your listing ending during it.
Think about the likely buyers for your items and when they will be likely to be online.