when using a multi checkbox in php, the name of the checkbox must end in "[]", for example:
<input type="checkbox" name="Availability[]" value="Monday" id="Availability_0" />
this will create an array of the multiple selections. when you add the binding in the insert or update record server behavior, code will look like:
<?php echo((isset($_POST["Availability[]"]))?$_POST["Availability[]"]:"") ?>
you will need to edit this code to use the php implode function to convert the array to a coma separated list
<?php echo ((isset($_POST["Availability"]))?implode(", ", $_POST["Availability"]):""); ?>
On the update page, you will need to make a few modifications to preselect the correct checkboxes.
You will need to use the php explode and in_array functions:
<input type="checkbox" name="Availability[]" value="Monday" id="Availability_0" <?php echo((in_array("Monday", explode(", ", $row_RecordsetName['ColumnName'])))?"checked=checked":""); ?>/>