This is how the process would work. You would have DT Register as the sign up platform. People register for the event throughout the month. In Acajoom Pro, you would build a notification newsletter and schedule it for the 20th. Sometime before the newsletter runs, you would use Event Mail to import the attendees from DT Register to the subscriber list. When the Acajoom mailing hits the Time and Date for mailing, then everyone gets notified. You can reuse the list to mail others, you can combine multiple attendees from different lists on one email, and you can import the attendees to Community Builder to make them site members.