Puedes especificar que es una tabla fija con el style="table-layout:fixed;"
y luego especificar el ancho de cada columna con los tags "col".
Mira el ejemplo, y si no cabes CSS te recomiendo que aprendas, te será muy útil:
<html>
<head>
<style type="text/css">
.tablaDias { table-layout:fixed; }
td { padding: 2px 2px 2px 2px; text-align-center; }
.columnaDia { width: 40px; }
</style>
</head>
<body>
<table border="1">
<tr>
<td valign="middle" >Gen</td>
<td>
<!--
<table cellspacing="1" cellpadding="0" border="1">
-->
<table class="tablaDias">
<col class="columnaDia"/>
<col class="columnaDia"/>
<col class="columnaDia"/>
<col class="columnaDia"/>
<col class="columnaDia"/>
<col class="columnaDia"/>
<col class="columnaDia"/>
<tr>
<td >Dl</td>
<td >Dm</td>
<td >Dc</td>
<td >Dj</td>
<td >Dv</td>
<td >Ds</td>
<td >Du</td>
</tr>
<tr>
<td >1</td>
<td >2</td>
<td >3</td>
<td >4</td>
<td >5</td>
<td >6</td>
<td >7</td>
</tr>
<tr>
<td >08:00</td>
<td >08:00</td>
<td ></td>
<td >08:00</td>
<td ></td>
<td ></td>
<td ></td>
</tr>
</table>
</td>
</tr>
<tr>
<td valign="middle" >Feb</td>
<td>
<!--
<table cellspacing="1" cellpadding="0" border="1">
-->
<table class="tablaDias">
<col class="columnaDia"/>
<col class="columnaDia"/>
<col class="columnaDia"/>
<col class="columnaDia"/>
<col class="columnaDia"/>
<col class="columnaDia"/>
<col class="columnaDia"/>
<tr>
<td >Dl</td>
<td >Dm</td>
<td >Dc</td>
<td >Dj</td>
<td >Dv</td>
<td >Ds</td>
<td >Du</td>
</tr>
<tr>
<td ></td>
<td ></td>
<td ></td>
<td >1</td>
<td >2</td>
<td >3</td>
<td >4</td>
</tr>
<tr>
<td ></td>
<td ></td>
<td ></td>
<td >08:00</td>
<td ></td>
<td ></td><td ></td>
</tr>
</table>
</td>
</tr>
</table>
</body>
</html>