Well, if you want to keep using $_POST vars, you'll need to set the $scroll variable as a hidden post var
PHP Code:
<?php
// set scroll equal to scroll + increment
$scroll = $_POST['scroll'] + $_POST['increment'];
?>
(code displaying the calendar table...)
<form action="reservations.php" method=post>
Scroll Forward by
<select name="increment">
<option value="1">1 Week</option>
<option value="2">2 Weeks</option>
<option value="3">3 Weeks</option>
<option value="4">4 Weeks</option>
</select>
<input type="hidden" name="scroll" value="<?=$scroll>">
<input type="submit" name="scroll_forward" value="Go">
</form>