body {
    max-width: 900px;
}

section.calendar-container {
    display: flex;
    flex-wrap: wrap;
}
section.calendar-container > h2 {
    min-width: 93%;
}
.calendar-month {
    margin: 1em;
    width: minmax(300px, 45%);
}
.calendar-month * {
  text-align: center;
}