GET myindex/_search?
{
 "size":0,
 "query": {
    "bool": {
      "must": [
        { "match": {
          "PersonID": "111"
        }},
        { "match": {
          "Month": "5"
        }}
      ]
    } },
   "aggs": {
    "stay_days": {
     "terms" : {
      "field": "Month"
     }
    }
   }
}

{
  "size": 0,
  "query": {
    "bool": {
      "must": [
        {
          "match": {
            "PersonID": "111"
          }
        },
        {
          "range": {
            "datetime": {
              "gte": "2017-05-01",
              "lt": "2017-06-01"
            }
          }
        }
      ]
    }
  },
  "aggregations": {
    "my_day_histogram": {
      "date_histogram": {
        "field": "datetime",
        "interval": "1d",
        "min_doc_count": 1
      }
    }
  }
}

"aggregations": {
    "my_month_histogram": {
      "date_histogram": {
        "field": "first_timestamp",
        "interval": "1M",
        "min_doc_count": 1
      },
      "aggregations": {
        "my_day_histogram": {
          "date_histogram": {
            "field": "first_timestamp",
            "interval": "1d"
          }
        }
      }
    }
  }