Ошибка в проверке задания.
<!DOCTYPE>
<html>
<head>
<title>Grid Calendar</title>
<style>
body {
margin: 0;
}
h1 {
font-family: Tahoma;
font-style: italic;
text-align: center;
}
.grid-container {
grid-template-columns: repeat (7, auto);
display: grid;
grid-auto-rows: 4vh;
gap: 1vw;
height: 100vh;
width: 100vw;
}
.grid-item {
background-color: rgb(255, 198, 0);
padding: 1vh 2vw;
font-size: 2vh;
font-style: italic;
font-family: Tahoma;
text-align: center;
}
.header {
font-weight: bold;
}
.month-start {
grid-column-start: 4;
}
</style>
</head>
<body>
<h1>December</h1>
<div class="grid-container">
<div class="grid-item header">Sunday</div>
<div class="grid-item header">Monday</div>
<div class="grid-item header">Tuesday</div>
<div class="grid-item header">Wednesday</div>
<div class="grid-item header">Thursday</div>
<div class="grid-item header">Friday</div>
<div class="grid-item header">Saturday</div>
<div class="grid-item month-start">1</div>
<div class="grid-item">2</div>
<div class="grid-item">3</div>
<div class="grid-item">4</div>
<div class="grid-item">5</div>
<div class="grid-item">6</div>
<div class="grid-item">7</div>
<div class="grid-item">8</div>
<div class="grid-item">9</div>
<div class="grid-item">10</div>
<div class="grid-item">11</div>
<div class="grid-item">12</div>
<div class="grid-item">13</div>
<div class="grid-item">14</div>
<div class="grid-item">15</div>
<div class="grid-item">16</div>
<div class="grid-item">17</div>
<div class="grid-item">18</div>
<div class="grid-item">19</div>
<div class="grid-item">20</div>
<div class="grid-item">21</div>
<div class="grid-item">22</div>
<div class="grid-item">23</div>
<div class="grid-item">24</div>
<div class="grid-item">25</div>
<div class="grid-item">26</div>
<div class="grid-item">27</div>
<div class="grid-item">28</div>
<div class="grid-item">29</div>
<div class="grid-item">30</div>
<div class="grid-item">31</div>
</div>
</body>
</html>