First you need to set value on your options:
<select title="Vacances scolaires" id="zones" class="selectpicker" data-width="fit"
multiple="" style="display: none;" name="vacances[]">
<option value='all' selected>All zones</option>
<option data-icon="glyphicon glyphicon-zone-a" value="zone_a">Zone A</option>
<option data-icon="glyphicon glyphicon-zone-b" value="zone_b">Zone B</option>
<option data-icon="glyphicon glyphicon-zone-c" value="zone_c">Zone C</option>
</select>
Then you need to subscribe to your selectpicker:
$('#zones').on('change',function(){
$('#calendar').fullCalendar('rerenderEvents');
});
And finally you need to check if your selection includes your class or all:
eventRender: function(event, element) {
const zone_class = event.className.toString();
const selectedValues = $('#zones').val();
return selectedValues.includes('all')
|| selectedValues.includes(zone_class);
}
Check this Plunker.