The first cast, as the name suggests, is the day when your engagement program's stream casts the nurture content for the first time and later, it casts the nurture content as per the set cadence. It's important that the first cast date coincides with the day of the week you choose in cadence; otherwise, the content will go out twice in the first week (on the first cast date and on the day of the week, as per the set cadence).
Also, AFAIK - if you try updating the stream cadence, it won't allow you to set the cadence with the date in the past - hence if you're updating the cadence of the stream that has been running, you should also make sure that you update the first cast date to the date you'd first like to send the stream content. After the first cast, the stream would stick to the cadence that you'd set, e.g., weekly 1x on Wednesdays at 8 AM.
I hope you find this helpful and let us know if you have any questions. 🙂