code: function setKokuminNoKyujitu(year, holidays) {
    if (year < 1988) {
      return;
    }
    var kokumin = [];
    var last = null;
    Object.keys(holidays).forEach(md => {
      var date = new Date(year, md.slice(0, -2) * 1 - 1, md.slice(-2) * 1);
      if (last){
        last.setTime(last.getTime() + A_DAY);
        if (last.getTime() + A_DAY === date.getTime()) {
          kokumin.push((last.getMonth() + 1) * 100 + last.getDate());
        }
      }
      last = date;
    });
    // 他の祝日や振替休日が優先される
    kokumin.forEach(x => holidays[x] = holidays[x] || '国民の休日');
  }
docs: 国民の休日を設定する
  施行: 1988-
  @method setKokuminNoKyujitu
  @param  {Number}   year
  @param  {Object}   holidays