n8n Workflow: Backfill Canceled Appointments with Google Calendar + WhatsApp

Backfill canceled appointments automatically with Google Calendar + WhatsApp

This n8n workflow detects canceled or no-show appointments in your Google Calendar, matches the newly freed time slots to your Google Sheets waitlist, and (optionally) sends WhatsApp offers—all while logging every offer back to Sheets.

What this workflow does

• Runs every 15 minutes on a schedule to keep backfilling timely.
• Fetches upcoming Google Calendar events within a configured lookahead window, including deleted/cancelled events.
• Reads your Google Sheets Bookings and Waitlist tabs to understand what’s booked and who’s waiting.
• Compares calendar events vs. Bookings to identify freed slots from:
  • cancelled/deleted appointments
  • bookings marked as no-show
• Enforces a minimum notice window by excluding slots that fall inside your configured cutoff period.
• Matches freed slots to waitlist entries in join order, with optional filtering by requested service and/or practitioner.
• Drafts one or more offers per slot for eligible waitlist entries.
• Optional WhatsApp sending + logging: if enabled, sends each offer via WhatsApp and appends offer details to an Offer log sheet; otherwise runs as a dry run without sending.

Use cases

• Reduce downtime after last-minute cancellations by immediately offering openings to people on your waitlist.
• Backfill no-shows by turning missed appointments into opportunities for waitlisted clients.
• Support multiple services/practitioners while only notifying the right waitlist segment.

Technical details

• Scheduler: runs every 15 minutes.
• Integrations: Google Calendar, Google Sheets, and WhatsApp Business Cloud.
• Workflow logic uses: if, set, code, no op, split out, and WhatsApp.
• Setup requires: Calendar ID, spreadsheet ID + required tabs/columns (Bookings, Waitlist, Offer log), and WhatsApp Phone Number ID (with waitlist numbers in WhatsApp-compatible format).