The onBeforeTimeHeaderRender event is fired before a Gantt time header cell is rendered. You can use it to customize the time header cells.
DayPilot.Gantt.onBeforeTimeHeaderRender(args)
gantt.onBeforeTimeHeaderRender = function(args) { if (args.header.start.getDayOfWeek() === 6) { args.header.html = "Sat"; } };