How to write a Elastic Aggregation on multiple fields

I'm having a requirement to aggregate the Elastic Search on multiple fields.

I have a nested field called Working days and below is the mapping for the same.

"workingDays": {
	"type": "nested",
	"properties": {
		"mon": {
			"type": "nested",
			"properties": {
				"availability": {
					"type": "boolean"
				},
				"title": {
					"type": "text",
                    "fields": {
                        "keyword": {
                           "type": "keyword"
                         }
                    }
				},
				"notes": {
					"type": "text"
				}
			}
		},
        "tue": {
			"type": "nested",
			"properties": {
				"availability": {
					"type": "boolean"
				},
				"title": {
					"type": "text",
                    "fields": {
                        "keyword": {
                           "type": "keyword"
                         }
                    }
				},
				"notes": {
					"type": "text"
				}
			}
		}
	}
}

In this way, the mapping goes on for all the days.

Now, the data is shown as below

"workingdays": {
		"mon": {
			"availability": true,
			"title": "Monday",
			"notes": ""
		},
		"tue": {
			"title": "Tuesday",
			"notes": "On Tuesdays, drop off and pick up times must be scheduled between 12pm to 2pm",
			"availability": false
		}
	}

So, my requirement is as follows. I need to aggregate and result in all the data which are available on specific days, something similar to this.

"buckets": [
   {
	  "key": "Monday",
	  "doc_count": 3
   },
   {
	  "key": "Tuesday",
	  "doc_count": 5
   }
]

I have tried nested aggregation, composition aggregation, and multi-field aggregation but none of them resulted in a usable format.

You can use nested aggregations inside of filter aggregation inside of nested aggregation :).

PUT test_days
{
  "mappings": {
    "properties": {
      "workingdays": {
        "type": "nested",
        "properties": {
          "mon": {
            "type": "nested",
            "properties": {
              "availability": {
                "type": "boolean"
              },
              "title": {
                "type": "text",
                "fields": {
                  "keyword": {
                    "type": "keyword"
                  }
                }
              },
              "notes": {
                "type": "text"
              }
            }
          },
          "tue": {
            "type": "nested",
            "properties": {
              "availability": {
                "type": "boolean"
              },
              "title": {
                "type": "text",
                "fields": {
                  "keyword": {
                    "type": "keyword"
                  }
                }
              },
              "notes": {
                "type": "text"
              }
            }
          }
        }
      }
    }
  }
}

POST test_days/_doc?refresh
{
  "workingdays": {
    "mon": {
      "availability": true,
      "title": "Monday",
      "notes": ""
    },
    "tue": {
      "title": "Tuesday",
      "notes": "On Tuesdays, drop off and pick up times must be scheduled between 12pm to 2pm",
      "availability": false
    }
  }
}

GET test_days/_search
{
  "size": 0,
  "aggs": {
    "total": {
      "nested": {
        "path": "workingdays"
      },
      "aggs": {
        "monday": {
          "filters": {
            "filters": {
              "monday_availability": {
                "nested": {
                  "path": "workingdays.mon",
                  "query": {
                    "term": {
                      "workingdays.mon.availability": {
                        "value": true
                      }
                    }
                  }
                }
              },
              "tuesday_availability": {
                "nested": {
                  "path": "workingdays.tue",
                  "query": {
                    "term": {
                      "workingdays.tue.availability": {
                        "value": true
                      }
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}

Note: case sensitive!! double check if you are using workingDays or workingdays.