How are you testing the automation?
The trigger variable exists only if your automation was triggered by its Calendar Trigger. If you use another means to test it (the Run command or automation.trigger service call) the trigger variable will be undefined.
Reference: Testing your automation
alias: Work day
description: ""
trigger:
- platform: calendar
event: start
offset: "0:0:0"
entity_id: calendar.rhs_sy22_23
condition:
- condition: template
value_template: "{{ 'workday' in trigger.calendar_event.summary }}"
action:
- service: notify.persistent_notification
data:
message: "It's a workday."
mode: single