That's right, if you want to send it from Monday to Friday you can do so through the way you described. By the day field, there is a 'Select time' section, where you can specify the time during those days you would like the email to be sent. This time will be set up based on your default instance time zone, in our case you can see it is GMT. You mentioned that you need to send emails based on different time zones, so this won't work very good for you. It's quite a limitation. If you have Workspaces and Partitions you could set different default times. If not I would suggest you calculate a time that might be appropriate for your different time zones and apply it.
Hope this helps you.
Yes, this is well documented.
Duration=1 month or X days
Must End - M-F
If Month ends on Saturday, then it waits until the next Monday at 9am.