spreadsheet_slack.gs

function sendMessage(text){
  var postUrl = 'URL';
  var username = 'bot';
  var icon = ':hatched_chick:';
  var message = text;

  var jsonData = {
     "username" : username,
     "icon_emoji": icon,
     "text" : message
  };
  var payload = JSON.stringify(jsonData);

  var options =
  {
    "method" : "post",
    "contentType" : "application/json",
    "payload" : payload
  };

  UrlFetchApp.fetch(postUrl, options);
}

function compareDate(date1,date2){
  if(date1.getFullYear() === date2.getFullYear() && date1.getMonth() === date2.getMonth() && date1.getDate() === date2.getDate()){
    return true;
  }else{
    return false;
  }
}

function myFunction(){
  var spreadSheet = SpreadsheetApp.openByUrl('url');
  var sheet = spreadSheet.getSheets()[0];

  var date_array = sheet.getRange(2, 2, 51).getValues();
  var name_array = sheet.getRange(2, 3, 51).getValues();
  
  var today = new Date();
  date_array.some(function(array, i, date_array){
    if(compareDate(array[0], today)==true && name_array[i][0].length > 0){
      var date = today.getFullYear() + '/' + (today.getMonth() + 1) + '/' + today.getDate()
      var message = date + 'の係は' + name_array[i][0] + 'さんだよ';
      sendMessage(message);
    }
  });
}