In your selector, you need a more specific specificity, as it ui-datepicker-week-endis in <td>, but there <a>with, ui-state-defaultinside, for example:
.ui-datepicker-week-end, .ui-datepicker-week-end a.ui-state-default {color:red;}
Here you can test it . If you're interested, the markup for the day cell is as follows:
<td class="ui-datepicker-week-end">
<a class="ui-state-default" href="#">DAY NUMBER</a>
</td>