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This file isn't connected to a database yet. Open it in a text editor, find the firebaseConfig block near the top of the script, and paste in the config from your Firebase project (Project settings → Your apps → Web app). Then re-upload it to your host.
Full step-by-step instructions are in the setup guide that came with this file.
Attendance rates count only these days. To cancel or add a single date, use the link under the date on the Daily tab.