It really depends on your own time zone and the time zone(s) of the countries your best converting traffic comes from.
Removing the time zone aspect, I've found a lot of people buy memberships between 4 PM and 11 PM wherever they are located. So it looks like after work and before bed are the best times to get people to part with their money.
Weekends are a bit more flexible....I guess some people like to wake up and treat themselves to something special.
