<div class="calendar-container">
  <div class="calendar-header">
    <h1>
       12月
      <button>▾</button>
    </h1>
    <p>2018</p>
  </div>
  <div class="calendar"><span class="day-name">月</span><span class="day-name">火</span><span class="day-name">水</span><span class="day-name">木</span><span class="day-name">金</span><span class="day-name blue">土</span><span class="day-name red">日</span>
    <div class="day day--disabled">30</div>
    <div class="day day--disabled">31</div>
    <div class="day">1</div>
    <div class="day">2</div>
    <div class="day">3</div>
    <div class="day">4</div>
    <div class="day">5</div>
    <div class="day">6</div>
    <div class="day">7</div>
    <div class="day">8</div>
    <div class="day">9</div>
    <div class="day">10</div>
    <div class="day">11</div>
    <div class="day">12</div>
    <div class="day">13</div>
    <div class="day">14</div>
    <div class="day">15</div>
    <div class="day">16</div>
    <div class="day">17</div>
    <div class="day">18</div>
    <div class="day">19</div>
    <div class="day">20</div>
    <div class="day">21</div>
    <div class="day">22</div>
    <div class="day">23</div>
    <div class="day">24</div>
    <div class="day">25</div>
    <div class="day">26</div>
    <div class="day">27</div>
    <div class="day">28</div>
    <div class="day">29</div>
    <div class="day">30</div>
    <div class="day">31</div>
    <div class="day day--disabled">1</div>
    <div class="day day--disabled">2</div>
    <section class="task task--warning">プロジェクト見積もり</section>
    <section class="task task--danger">デザイン・打ち合わせ</section>
    <section class="task task--primary">コーディング
      <div class="task__detail">
        <h2>担当</h2>
        <p>田中・高橋・鈴木</p>
      </div>
    </section>
    <section class="task task--info">ローンチ</section>
  </div>
</div>
html,
body {
  width: 100%;
  height: 100%;
}

body {
  background: #f5f7fa;
  padding: 40px 0;
  box-sizing: border-box;
  font-family: Montserrat, "sans-serif";
  color: #51565d;
}

.calendar {
  display: grid;
  width: 100%;
  grid-template-columns: repeat(7, minmax(20px, 1fr));
  grid-template-rows: 50px;
  grid-auto-rows: 120px;
  overflow: auto;
}
.calendar-container {
  width: 90%;
  margin: auto;
  overflow: hidden;
  box-shadow: 0 2px 20px rgba(0, 0, 0, 0.1);
  border-radius: 10px;
  background: #fff;
  max-width: 600px;
}
.calendar-header {
  text-align: center;
  padding: 20px 0;
  background: linear-gradient(to bottom, #fafbfd 0%, rgba(255, 255, 255, 0) 100%);
  border-bottom: 1px solid rgba(166, 168, 179, 0.12);
}
.calendar-header h1 {
  margin: 0;
  font-size: 18px;
}
.calendar-header p {
  margin: 5px 0 0 0;
  font-size: 13px;
  font-weight: 600;
  color: rgba(81, 86, 93, 0.4);
}
.calendar-header button {
  background: 0;
  border: 0;
  padding: 0;
  color: rgba(81, 86, 93, 0.7);
  cursor: pointer;
  outline: 0;
}

.day {
  border-bottom: 1px solid rgba(166, 168, 179, 0.12);
  border-right: 1px solid rgba(166, 168, 179, 0.12);
  text-align: right;
  padding: 14px 20px;
  letter-spacing: 1px;
  font-size: 12px;
  box-sizing: border-box;
  color: #98a0a6;
  position: relative;
  pointer-events: none;
  z-index: 1;
}
.day:nth-of-type(7n + 7) {
  border-right: 0;
}
.day:nth-of-type(n + 1):nth-of-type(-n + 7) {
  grid-row: 2;
}
.day:nth-of-type(n + 8):nth-of-type(-n + 14) {
  grid-row: 3;
}
.day:nth-of-type(n + 15):nth-of-type(-n + 21) {
  grid-row: 4;
}
.day:nth-of-type(n + 22):nth-of-type(-n + 28) {
  grid-row: 5;
}
.day:nth-of-type(n + 29):nth-of-type(-n + 35) {
  grid-row: 6;
}
.day:nth-of-type(7n + 1) {
  grid-column: 1/1;
}
.day:nth-of-type(7n + 2) {
  grid-column: 2/2;
}
.day:nth-of-type(7n + 3) {
  grid-column: 3/3;
}
.day:nth-of-type(7n + 4) {
  grid-column: 4/4;
}
.day:nth-of-type(7n + 5) {
  grid-column: 5/5;
}
.day:nth-of-type(7n + 6) {
  grid-column: 6/6;
}
.day:nth-of-type(7n + 7) {
  grid-column: 7/7;
}
.day-name {
  font-size: 12px;
  text-transform: uppercase;
  color: #99a1a7;
  text-align: center;
  border-bottom: 1px solid rgba(166, 168, 179, 0.12);
  line-height: 50px;
  font-weight: 500;
}
.day--disabled {
  color: rgba(152, 160, 166, 0.6);
  background-color: #ffffff;
  background-image: url("data:image/svg+xml,%3Csvg width='40' height='40' viewBox='0 0 40 40' xmlns='http://www.w3.org/2000/svg'%3E%3Cg fill='%23f9f9fa' fill-opacity='1' fill-rule='evenodd'%3E%3Cpath d='M0 40L40 0H20L0 20M40 40V20L20 40'/%3E%3C/g%3E%3C/svg%3E");
  cursor: not-allowed;
}

.task {
  border-left-width: 3px;
  padding: 8px 12px;
  margin: 10px;
  border-left-style: solid;
  font-size: 14px;
  position: relative;
}
.task--warning {
  border-left-color: #fdb44d;
  grid-column: 4 / span 3;
  grid-row: 3;
  background: #fef0db;
  align-self: center;
  color: #fc9b10;
  margin-top: -5px;
}
.task--danger {
  border-left-color: #fa607e;
  grid-column: 2 / span 3;
  grid-row: 3;
  margin-top: 15px;
  background: rgba(253, 197, 208, 0.7);
  align-self: end;
  color: #f8254e;
}
.task--info {
  border-left-color: #4786ff;
  grid-column: 6 / span 2;
  grid-row: 5;
  margin-top: 15px;
  background: rgba(218, 231, 255, 0.7);
  align-self: end;
  color: #0a5eff;
}
.task--primary {
  background: #4786ff;
  border: 0;
  border-radius: 4px;
  grid-column: 3 / span 3;
  grid-row: 4;
  align-self: end;
  color: #fff;
  box-shadow: 0 10px 14px rgba(71, 134, 255, 0.4);
}
.task__detail {
  position: absolute;
  left: 0;
  top: calc(100% + 10px);
  background: #fff;
  border: 1px solid rgba(166, 168, 179, 0.2);
  color: #000;
  padding: 20px;
  box-sizing: border-box;
  border-radius: 4px;
  box-shadow: 0 10px 40px rgba(0, 0, 0, 0.08);
  z-index: 2;
}
.task__detail:after, .task__detail:before {
  bottom: 100%;
  left: 30%;
  border: solid transparent;
  content: " ";
  height: 0;
  width: 0;
  position: absolute;
  pointer-events: none;
}
.task__detail:before {
  border-bottom-color: rgba(166, 168, 179, 0.2);
  border-width: 8px;
  margin-left: -8px;
}
.task__detail:after {
  border-bottom-color: #fff;
  border-width: 6px;
  margin-left: -6px;
}
.task__detail h2 {
  font-size: 15px;
  margin: 0;
  color: #51565d;
}
.task__detail p {
  margin-top: 4px;
  font-size: 12px;
  margin-bottom: 0;
  font-weight: 500;
  color: rgba(81, 86, 93, 0.7);
}

.blue{color:blue;}
.red{color:red;}

External CSS

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