HA - Use Calendar list service to define binary sensor and populate attribute.

calendar_list_to_binary.yaml

# Script
alias: Calendar 1-Day Agenda to Event
sequence:
  - service: calendar.list_events
    data:
      duration:
        hours: 24
        minutes: 0
        seconds: 0
    target:
      entity_id: calendar.my_calendar
    response_variable: agenda
  - event: custom_calendar_agenda
    event_data:
      calendar: my_calendar
      cal_events: "{{ agenda }}"
mode: single


## Template binary sensor:

template:
  - trigger:
      - platform: event
        event_type: custom_calendar_agenda
        event_data:
          calendar: my_calendar
    binary_sensor:
      - name: Calendar Agenda
        unique_id: custom_calendar_agenda_001
        state: "{{ trigger.event.data.cal_events['events'] | count > 0 }}"
        attributes: 
          events: "{{ trigger.event.data.cal_events['events'] }}"

ok, so 1st part goes into scripts.yaml? where does the 2nd part go?

It can go directly in configuration.yaml or any properly split file setup for the template integration (https://www.home-assistant.io/docs/configuration/splitting_configuration/)