Searching Elasticsearch for first occurance of an event each day

We use Logstash to receive logs, pass to Elasticsearch, and browse using Kibana. A very common setup.

One of the fields in each entry is @timestamp, with an example content of 03/18/2015 18:02:52. What filter should I use to display only the first entry of each day?

I don't believe you can do this with a filter - being first in a day isn't a property you can ascertain by looking at a single document. You should however be able to do this with an aggregation: first aggregate using a date_histogram with interval day, to group the events by day. Then use the top_hits aggregation to pull out one result per day (requires elasticsearch 1.3 or higher). Your query should look like this

{
  "query": {
    "match_all": {}
  },
  "aggs": {
    "by-day": {
      "date_histogram": {
        "field": "timestamp",
        "interval": "day"
      },
      "aggs": {
        "top_for_day": {
          "top_hits": {
            "size": 1,
            "sort": [
              {
                "timestamp": {
                  "order": "asc"
                }
              }
            ]
          }
        }
      }
    }
  }
}

Which should produce results like (trimmed slightly for brevity)

{
  "aggregations": {
    "by-day": {
      "buckets": [
        {
          "key_as_string": "2015-02-01T00:00:00.000Z",
          "key": 1422748800000,
          "doc_count": 7635,
          "top_for_day": {
            "hits": {
              "total": 7635,
              "max_score": null,
              "hits": [
                {
                  "_index": "events-2015-02",
                  "_type": "event",
                  "_id": "c64f85ac-a870-441f-bedb-e24db47fd02a",
                  "_score": null,
                  "_source": {
                    "eventTime": "2015-02-01T00:00:26Z"
                  },
                  "sort": [
                    1422748826000
                  ]
                }
              ]
            }
          }
        },
        {
          "key_as_string": "2015-02-02T00:00:00.000Z",
          "key": 1422835200000,
          "doc_count": 8182,
          "top_for_day": {
            "hits": {
              "total": 8182,
              "max_score": null,
              "hits": [
                {
                  "_index": "events-2015-02",
                  "_type": "event",
                  "_id": "c544278d-9f51-41a8-827b-9c70c0a057ca",
                  "_score": null,
                  "_source": {
                    "timestamp": "2015-02-02T00:00:19Z"
                  },
                  "sort": [
                    1422835219000
                  ]
                }
              ]
            }
          }
        }
      ]
    }
  }
}