One of the ways to help with a Join Day is to have clear onboarding paths for that particular subproject or team.
One of the first tasks for our soon-to-be Outreachy Intern will be establishing onboarding badge series' for each team and subproject! As an example of this, we have the "bootstrapped" series, which is in progress, but includes a number of common badges that help to introduce new contributors generally to a number of aspects of the project. (See: https://fedorahosted.org/fedora-badges/ticket/409
Even if we don't have a badge series identified yet, having the steps listed to join a particular group will help with both creating the series, and engaging with new contributors in an event like a Join-Day.