Hi Rebekah,
Thanks so much for reaching out!
You should be able to configure the Countdown Widget to countdown for a recurring event. You shouldn’t actually need the slug or post ID at all. I have configured something like the following and a countdown begins in the sidebar:

As a first troubleshooting step, could you please provide us with your complete system information in a private reply using the instructions found in the following link?
https://theeventscalendar.com/knowledgebase/sharing-sys-info/
Let me know if anything else comes up in the meantime!
Thanks,
Jaime