code: function getShunbun(year) {
    if (year < 1949 || 2099 < year) {
      return null;
    }
    var day;
    switch(year % 4) {
    case 0:
      day = year <= 1956 ? 21 : year <= 2088 ? 20 : 19;
      break;
    case 1:
      day = year <= 1989 ? 21 : 20;
      break;
    case 2:
      day = year <= 2022 ? 21 : 20;
      break;
    case 3:
      day = year <= 1923 ? 22 : year <= 2055 ? 21 : 20;
    }
    return [3, day, '春分の日'];
  }
docs: 春分の日 (wikiの簡易計算より)
  @method getShunbun
  @param  {Number}   year
  @return {Array}    shunbun