$("#available_classes_calendar").fullCalendar({ header: { left : 'prev,next', center : 'title' }, defaultView: 'agendaWeek', allDaySlot: false, minTime: "06:30:00", maxTime: "24:00:00", slotDuration: "06:00:01" });
As for displaying 06:30, 07:30, and so on, along the vertical axis, you need to set the slotDuration: "06:30:01". Look at jsfiddle
why we need "+01" seconds you can see in the original code the line 5714 to 5719
((!slotNormal || !minutes) ? // if irregular slot duration, or on the hour, then display the time '<span>' + // for matchCellWidths htmlEscape(slotDate.format(this.axisFormat)) + '</span>' : ''
Now $ slotNormal is determined by line 5701:
var slotNormal = this.slotDuration.asMinutes() % 15 === 0;
You can find the answer by clicking on the answer link.