Используйте второй, <div>
а затем вы можете делать все, что хотите. Например:
<table>
<thead>
<tr>
<th style="height:10px; width:100px">Monday</th>
<th style="height:10px; width:100px">Tuesday</th>
<th style="height:10px; width:100px">Wednesday</th>
<th style="height:10px; width:100px">Thursday</th>
<th style="height:10px; width:100px">Friday</th>
<th style="height:10px; width:100px">Saturday</th>
<th style="height:10px; width:100px">Sunday</th>
</tr>
</thead>
<tbody>
<tr>
<td><div style="height:100px; width:100px; background-color:red; ">1
<div style="height:10px; width:1000px; background-color:green;"></div>
</div></td>
<td><div style="height:100px; width:100px">2</div></td>
<td><div style="height:100px; width:100px">3</div></td>
<td><div style="height:100px; width:100px">4</div></td>
<td><div style="height:100px; width:100px">5</div></td>
<td><div style="height:100px; width:100px">6</div></td>
<td><div style="height:100px; width:100px">7</div></td>
</tr>
<tr>
<td><div style="height:100px; width:100px">
8
<div style="height:10px; width:1000px; background-color:green;"></div>
</div></td>
<td><div style="height:100px; width:100px">9</div></td>
<td><div style="height:100px; width:100px">10</div></td>
<td><div style="height:100px; width:100px">11</div></td>
<td><div style="height:100px; width:100px">12</div></td>
<td><div style="height:100px; width:100px">13</div></td>
<td><div style="height:100px; width:100px">14</div></td>
</tr>
</tbody>
</table>
<script></script>