SonicMailer Pro's Send Scheduler plugin gives you the ability to create a message now and choose to have it automatically sent at some time in the future. This would allow you to prepare your messages when you have the time to do so, and have them send out later in the week.

Send Scheduler makes it very easy to schedule a message for sending from the message queue page:

How to Order

• New Customers
  To order the Send Scheduler plugin, simply select the plugin on the order page when pruchasing your SonicMailer Pro license
• Existing Customers
  Existing customers can add the plugin to a "New Order" from within the client area