Average rainfall on one particular day over the last 30 years

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// import image collection
var chirps = ee.ImageCollection("UCSB-CHG/CHIRPS/DAILY")

// filter for Vietnam
var countries = ee.FeatureCollection('ft:1tdSwUL7MVpOauSgRzqVTOwdfy17KDbw-1d9omPw');
var country_names = ['Vietnam'];
var myCountry = countries.filter(ee.Filter.inList('Country', country_names));
var aoi = myCountry.geometry();

// select day of year and clip for country
var rainfall = chirps.filter(ee.Filter.calendarRange(200, 201, 'day_of_year')).mean().clip(aoi);

// add map
Map.addLayer(rainfall,{min:5,max:15,palette:"white,blue,darkblue,red,purple"},"mean daily prepcipitation");<span id="mce_SELREST_start" style="overflow:hidden;line-height:0;"></span>