JSFiddle - React, Tailwind, and code Playground

HTML

<div class="calendar">
    <div class="row">
        <div class="day">-</div>
        <div class="day">2018-05-01</div>
        <div class="day">2018-05-02</div>
        <div class="day">2018-05-03</div>
        <div class="day">2018-05-04</div>
        <div class="day">2018-05-05</div>
        <div class="day">2018-05-06</div>
        <div class="day">2018-05-07</div>
        <div class="day">2018-05-08</div>
        <div class="day">2018-05-09</div>
        <div class="day">2018-05-10</div>
    </div>
    <div class="row">
        <div class="day">Room #101</div>
        <div class="day" id="2018-05-01_1"></div>
        <div class="day" id="2018-05-02_1"></div>
        <div class="day" id="2018-05-03_1"></div>
        <div class="day" id="2018-05-04_1"></div>
        <div class="day" id="2018-05-05_1"></div>
        <div class="day" id="2018-05-06_1"></div>
        <div class="day" id="2018-05-07_1"></div>
        <div class="day" id="2018-05-08_1"></div>
        <div class="day" id="2018-05-09_1"></div>
        <div class="day" id="2018-05-10_1"></div>
    </div>
    <div class="row">
        <div class="day">Room #102</div>
        <div class="day" id="2018-05-01_2"></div>
        <div class="day" id="2018-05-02_2"></div>
        <div class="day" id="2018-05-03_2"></div>
        <div class="day" id="2018-05-04_2"></div>
        <div class="day" id="2018-05-05_2"></div>
        <div class="day" id="2018-05-06_2"></div>
        <div class="day" id="2018-05-07_2"></div>
        <div class="day" id="2018-05-08_2"></div>
        <div class="day" id="2018-05-09_2"></div>
        <div class="day" id="2018-05-10_2"></div>
    </div>
    <div class="row">
        <div class="day">Room #201</div>
        <div class="day" id="2018-05-01_3"></div>
        <div class="day" id="2018-05-02_3"></div>
        <div class="day" id="2018-05-03_3"></div>
        <div class="day" id="2018-05-04_3"></div>
        <div class="day" id="2018-05-05_3"></div>
        <div class="day"...

CSS

html, body {
  height: 100%;
}

.calendar {
  max-width: 90%;
  margin: 20px auto;
}

.row {
  display: flex;
  height: 40px;
}

.day {
  flex-grow: 1;
  flex-shrink: 1;
  flex-basis: 0;
  padding: 5px;
  box-shadow: -1px -1px #e2e2e2,  inset -1px -1px 0 0 #e2e2e2;
}