find and replace content based on a pattern

find and replace content based on a pattern

<tr>
<th></th>
<th class="ms-acal-month-top"><nobr><span>Sunday</span></nobr></th>
<th class="ms-acal-month-top"><nobr><span>Monday</span></nobr></th>
<th class="ms-acal-month-top"><nobr><span>Tuesday</span></nobr></th>
<th class="ms-acal-month-top"><nobr><span>Wednesday</span></nobr></th>
<th class="ms-acal-month-top"><nobr><span>Thursday</span></nobr></th>
<th class="ms-acal-month-top"><nobr><span>Friday</span></nobr></th>
<th class="ms-acal-month-top"><nobr><span>Saturday</span></nobr></th>
</tr>

i want the above to be replaced using jquery to 

<tr>
<th></th>
<th class="ms-acal-month-top"><nobr><span>S</span></nobr></th>
<th class="ms-acal-month-top"><nobr><span>M</span></nobr></th>
<th class="ms-acal-month-top"><nobr><span>T</span></nobr></th>
<th class="ms-acal-month-top"><nobr><span>W</span></nobr></th>
<th class="ms-acal-month-top"><nobr><span>T</span></nobr></th>
<th class="ms-acal-month-top"><nobr><span>F</span></nobr></th>
<th class="ms-acal-month-top"><nobr><span>S</span></nobr></th>
</tr>

thanks