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HTML 

              
                main
  table.calendar
    caption.calendar__banner--month
        h1 March
    thead
      tr
        th.calendar__day__header Sun
        th.calendar__day__header Mon
        th.calendar__day__header Tues
        th.calendar__day__header Wed
        th.calendar__day__header Thurs
        th.calendar__day__header Fri
        th.calendar__day__header Sat
    tbody
      tr
        td.calendar__day__cell
        td.calendar__day__cell
        td.calendar__day__cell
        td.calendar__day__cell 1
        td.calendar__day__cell 2
        td.calendar__day__cell 3
        td.calendar__day__cell 4
      tr
        td.calendar__day__cell 5
        td.calendar__day__cell 6
        td.calendar__day__cell 7
        td.calendar__day__cell 8
        td.calendar__day__cell 9
        td.calendar__day__cell 10
        td.calendar__day__cell 11
      tr
        td.calendar__day__cell 12
        td.calendar__day__cell 13
        td.calendar__day__cell 14
        td.calendar__day__cell(data-moon-phase="New Moon") 15
        td.calendar__day__cell 16
        td.calendar__day__cell 17
        td.calendar__day__cell 18
      tr
        td.calendar__day__cell 19
        td.calendar__day__cell 20
        td.calendar__day__cell 21
        td.calendar__day__cell 22
        td.calendar__day__cell 23
        td.calendar__day__cell 24
        td.calendar__day__cell 25
      tr
        td.calendar__day__cell 26
        td.calendar__day__cell 27
        td.calendar__day__cell 28
        td.calendar__day__cell(data-moon-phase="Full Moon") 29
        td.calendar__day__cell 30
        td.calendar__day__cell(data-bank-holiday="Good Friday") 31
        td.calendar__day__cell
!

CSS 

              
                @import url(https://fonts.googleapis.com/css?family=Montserrat:400,700);

$red: #e66053;
$black: #4E4F4A;
$cream: #F6E9DC;

$thin-border: 2px solid $red;

*, *::after, *::before {
  box-sizing: border-box;
}

body {
  background-color: $red;
  color: $black;
  display: flex;
  align-items: center;
  justify-content: center;
  font-family: 'Montserrat';
  font-weight: 700;
  min-height: 100vh;
}

main {
  background-color: $cream;
  box-shadow:
     0px 0px 0px 2px $red,
     10px 10px 20px 10px fade-out($black, .5);
  flex-basis: 980px;
}

.calendar {
  table-display: fixed;
  border: $thin-border;
  width: 100%;
}

.calendar__day__header,
.calendar__day__cell {
  border: $thin-border;
  text-align: center;
  width: 100% / 7; // fix width
  vertical-align: middle;
  &:first-child {
    border-left: none;
  }
  &:last-child {
    border-right: none;
  }
}

.calendar__day__header,
.calendar__day__cell {
  padding: .75rem 0 1.5rem;
}

.calendar__banner--month {
  border: $thin-border;
  border-bottom: none;
  text-align: center;
  padding: .75rem;
  h1 {
    background-color: $black;
    color: $cream;
    display: inline-block;
    font-size: 3rem;
    font-weight: 700;
    letter-spacing: 0.1em;
    padding: .5rem 2rem;
    text-transform: uppercase;
  }
}

.calendar__day__header {
  font-size: 1rem;
  letter-spacing: 0.1em;
  text-transform: uppercase;
}

.calendar__day__cell {
  font-size: 4rem;
  position: relative;
}

tr:nth-child(odd) > .calendar__day__cell:nth-child(odd) {
  color: $red;
}

tr:nth-child(even) > .calendar__day__cell:nth-child(even) {
  color: $red;
}

.calendar__day__cell[data-moon-phase] {
  background-color: $red;
  color: $black;
  &:after {
    content: attr(data-moon-phase);
    color: $cream;
    display: block;
    font-weight: 400;
    font-size: .75rem;
    position: absolute;
    bottom: 0;
    width: 100%;
    height: 1rem;
    text-transform: uppercase;
  }
}

.calendar__day__cell[data-bank-holiday] {
  background-color: $black;
  border-color: $black;
  
  border-bottom: none;
  color: $cream;
  &:after {
    content: attr(data-bank-holiday);
    color: $cream;
    display: block;
    font-size: .75rem;
    font-weight: 400;
    position: absolute;
    bottom: 0;
    width: 100%;
    height: 1rem;
    text-transform: uppercase;
  }
}
!

JS
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