Here's the version I use (fullCalendar v4):
datesRender: function(info) {
var list = $(info.el).find('.fc-list-table tbody');
list.find('.fc-list-heading').each((i,heading) => {
var children = $(heading).nextUntil('.fc-list-heading')
list.prepend(children)
list.prepend(heading)
})
},