You can make PowerMail work on your time frame by scheduling them to send at a later date. You can create and schedule your PowerMail and know that it will go out when you want, not just when you have the time to send it.
Go to the 'Publish' page of your PowerMail Issue and (1) Set 'Send When' to 'Send Later', (2) Enter in the date and time of the send, and (3) Select 'Send to Members'
Your PowerMail will now be sent at the time specified.
Any changes you make to the issue after scheduling will not appear in the issue that will be sent. To make changes you must unschedule and then reschedule it.