div.calendar header {
  display: flex;
  align-items: center;
  font-size: calc(16px + (26 - 16) * ((100vw - 300px) / (1600 - 300)));
  justify-content: center;
  margin-bottom: 2em;
  background: #000;
  color: #fff;
  min-height: 10vh;
  text-align: center;
}

ul {
  display: grid;
  grid-template-columns: repeat(7, 1fr);
  grid-gap: .125em;
  margin: 0 auto;
  padding: 0;
}

li {
  display: flex;
  align-items: center;
  justify-content: center;
  list-style: none;
  margin-left: 0;
}

ul.weekdays {
  margin-bottom: 1em;
}

ul.weekdays li {
  height: 4vw;
}

ul.day-grid li {
  background-color: #eaeaea;
}

ul.weekdays abbr[title] {
  border: none;
  font-weight: 800;
  text-decoration: none;
}

@media all and (max-width: 800px) {

  ul.weekdays li {
    font-size: 0;
  }

  ul.weekdays > li abbr:after {
      content: attr(title);
      font-size: calc(16px + (26 - 16) * ((100vw - 300px) / (1600 - 300)));
    text-align: center;
    }
}