n8n Workflow: Backfill Cancelled Slots w/ Google Calendar & WhatsApp

Backfill cancelled appointment slots automatically with Google Calendar + WhatsApp

This n8n workflow checks your Google Calendar every 15 minutes for cancelled or deleted appointments, finds the matching freed slots in your Google Sheet, and sends a WhatsApp offer to the right person from your Waitlist—so empty spaces don’t cost you revenue.

What this workflow does

• Runs on a 15-minute schedule to stay responsive without constant manual work.
• Reads appointment events from Google Calendar within your lookahead window (including deleted events).
• Loads Bookings and Waitlist tabs from the same Google Sheet (same spreadsheet, consistent data model).
• Marks an appointment row as a freed slot if the event was cancelled/deleted or if the booking is marked no-show.
• Excludes slots that start inside your minimum notice window.
• Matches freed slots to the first eligible waitlist member (joining order) who requested the relevant service and practitioner.
• WhatsApp sending is controlled by a switch: when sendOffers is on, offers are sent; when off, the workflow stops at a dry-run step and nothing goes out.
• Logs every sent offer by appending details (slot + person info) to the Offer log tab.

Use cases

• An appointment is cancelled at the last minute—this workflow immediately notifies the next eligible waitlist customer via WhatsApp.
• A “no-show” is recorded in your Bookings tab—freed capacity is offered to the earliest matching waitlist entry.
• You want a safe rollout: run with sendOffers off to verify matching and logging before enabling WhatsApp delivery.

Technical details

• Integrations: Google Calendar, Google Sheets, WhatsApp Business Cloud.
• n8n nodes/logic: schedule trigger, if, set, code, no op, split out, and whats app.
• Setup required: provide calendar ID + spreadsheet ID in Settings; create spreadsheet tabs named Bookings, Waitlist, and Offer log; store waitlist phone numbers with country code; add WhatsApp Business Cloud credentials and phone number ID.

Perfect for n8n users, automation engineers, and SaaS operators running appointment businesses who need reliable, data-driven backfilling of cancelled slots—without manual outreach.