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HTML 

              
                <main class="calendar">
  <div class="day one"></div>
  <div class="day two"></div>
  <div class="day three"></div>
  <div class="day four"></div>
  <div class="day five"></div>
  <div class="day six"></div>
  <div class="day seven"></div>
  <div class="tall-window">plants</div>
  <div class="day eight"></div>
  <div class="day nine"></div>
  <div class="day ten"></div>
  <div class="day eleven"></div>
  <div class="day twelve"></div>
  <div class="day thirteen"></div>
  <div class="day fourteen"></div>
  <div class="day fifteen"></div>
  <div class="day sixteen"></div>
  <div class="day seventeen"></div>
  <div class="day eighteen"></div>
  <div class="day nineteen"></div>
  <div class="day twenty"></div>
  <div class="day twentyone"></div>
  <div class="day twentytwo"></div>
  <div class="day twentythree"></div>
  <div class="day twentyfour"></div>
  <div class="day twentyfive"></div>
  <div class="day twentysix"></div>
  <div class="day twentysevent"></div>
  <div class="day twentyeight"></div>
  <div class="day twentynine"></div>
  <div class="day thirty"></div>
  <div class="day thirtyone"></div>
</main>
!

CSS 

              
                @use postcss-nested;

.calendar {
  --_card-size: calc(50% - 80px); 
  --_windows: 2;
  
  inline-size: min(80vw, 1200px);
  border: 5px solid black;
  position: relative;
  background: hsl(0 0% 90%);
  
  display: grid;
  grid-template-columns: repeat(var(--_windows), minmax(var(--_card-size), 1fr));  
  grid-auto-flow: dense;
  gap: 40px;
  padding: 40px;
  align-items: start;
  
  counter-reset: calendar;
  
  @media (min-width: 480px) {
    --_card-size: calc(33% - 80px);
    --_windows: 3;
  }
  
  @media (min-width: 720px) {
    --_card-size: calc(25% - 80px);
    --_windows: 4;
  }
  
  &::before {
    content: "";
    position: absolute;
    top: -30px;
    left: -5%;
    block-size: 20px;
    width: 110%;
    border: 5px solid black;
  }
  
  &::after {
    content: "";
    position: absolute;
    top: -60px;
    left: -10%;
    block-size: 25px;
    width: 120%;
    border: 5px solid black;
  }
}

.day {
  padding: 20px;
  box-shadow: 0 0 0 5px black;
  background: white;
  background-size: cover;
  background-position: center;
  min-block-size: 100%;
  position: relative;
  
  &::before {
    counter-increment: calendar;
    content: counter(calendar);
    
    position: absolute;
    bottom: -2ch;
    left: calc(50% - 2ch);
    
    background: white;
    display: grid;
    place-content: center;
    inline-size: 3ch;
    block-size: 3ch;
    border-radius: 1e3px;
    border: 3px solid black;
    font-size: .7rem;
  }
}

.one {
  border-radius: 1e2px 1e2px 0 0;
  background-image: url(https://doodleipsum.com/700x700/outline?1);
}

.two {
  aspect-ratio: 1;
  min-block-size: auto;
  align-self: center;
  background-image: url(https://doodleipsum.com/700x700/outline?2);
}

.three {
  border-radius: 1e2px;
  aspect-ratio: 1;
  min-block-size: auto;
  align-self: end;
  background-image: url(https://doodleipsum.com/700x700/outline?3);
}

.four {
  grid-row: span 2;
  background-image: url(https://doodleipsum.com/350x700/outline?4);
}

.five {
  aspect-ratio: 3/4;
  min-block-size: auto;
  align-self: center;
  background-image: url(https://doodleipsum.com/700x700/outline?5);
}

.six {
  border-radius: 20px 0 0;
  grid-column: span 2;
  background-image: url(https://doodleipsum.com/1400x700/outline?6);
}

.seven {
  background-image: url(https://doodleipsum.com/350x700/outline?7);
}

.eight {
  border-radius: 1e2px;
  aspect-ratio: 1;
  min-block-size: auto;
  align-self: end;
  background-image: url(https://doodleipsum.com/350x700/outline?8);
}

.nine {
  background-image: url(https://doodleipsum.com/350x700/outline?9);
}

.tall-window {
  grid-row: span 2;
  padding: 20px;
  box-shadow: 0 0 0 5px black;
  min-block-size: 100%;
}

.ten {
  background-image: url(https://doodleipsum.com/350x700/outline?10);
}

.eleven {
  border-radius: 1e2px;
  aspect-ratio: 1;
  min-block-size: auto;
  align-self: end;
  background-image: url(https://doodleipsum.com/350x700/outline?11);
}

* {
  box-sizing: border-box;
  margin: 0;
}

html {
  block-size: 100%;
}

body {
  min-block-size: 100%;
  font-family: system-ui, sans-serif;
  display: grid;
  place-content: center;
  padding-block-start: 20vh;
}
!

JS
!
999px

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