core/homeassistant/components/calendar/icons.json

{
  "entity_component": {
    "_": {
      "default": "mdi:calendar",
      "state": {
        "on": "mdi:calendar-check",
        "off": "mdi:calendar-blank"
      }
    }
  },
  "services": {
    "create_event": {
      "service": "mdi:calendar-plus"
    },
    "get_events": {
      "service": "mdi:calendar-month"
    }
  }
}