Displaying Clickable Grid

What's the best way to display a grid like below which will allow the user to select multiple boxes and then have the page capture the selected elements?

The existing code I am working with seems inadequate since it displays a grid of buttons where only one can be selected:

<tr>
<th>6-8 AM</th>
<td><button type="button" class="btn"></button></td>
<td><button type="button" class="btn"></button></td>
<td><button type="button" class="btn"></button></td>
<td><button type="button" class="btn"></button></td> 
<td><button type="button" class="btn"></button></td>     
<td><button type="button" class="btn"></button></td>  
<td><button type="button" class="btn"></button></td>                                   
</tr>