If you have added the jquery validation class to element, it always validated, if you don't want required you need to remove the class. For that you can use javascript function in checkbox
<input type="checkbox" value="true" onchange="set_validation()" name="Availability_MondayIsAvailable" id="Availability_MondayIsAvailable" checked="checked"> Monday
Add this function in your javascript
function set_validation() {
if ($('#Availability_MondayIsAvailable').is(':checked')) {
$( "#Availability_MondayStartTime" ).attr( "class", "validate[condRequired[Availability_MondayIsAvailable],custom[timeForm]]" );
$( "#Availability_MondayEndTime" ).attr( "class","validate[condRequired[Availability_MondayIsAvailable]]");
}
else {
$( "#Availability_MondayStartTime" ).attr( "class"," ");
$( "#Availability_MondayEndTime" ).attr( "class"," ");
}
}