table {
  table-layout: fixed;
  width: 1250px;
  height: 800px;
}

table td {
  height: 100px;
  position: relative;
}

table td:before {
  content: attr(data-day);
  display: block;
  position: absolute;
  top: 0;
  left: 0;
  background: yellow;
}