Hey Adam, each time there is a cast in an engagement program, the system will look from the top down and see if a person has received each piece of content. So, you're able to add content anywhere in the stream and if you added it as email #2 and someone is already on email #4, then they'll receive this email as the next cast. If someone new is added, then they'll receive email #1 and then your new email on the following cast.
With seasonal emails, you can also only make them available for a certain amount of time which is really convenient!