How to add "day of week" to the time header day

You can customize the calendar column headers using the onBeforeHeaderRender event handler:

const calendar = new DayPilot.Calendar("calendar", {
  // ...
  headerHeight: 50,
  onBeforeHeaderRender: args => {
      const date = args.header.start.toString("d MMMM");
      const dayOfWeek = args.header.start.toString("ddd");
      args.header.html = `${date}<br>${dayOfWeek}`;
  },
});

You may want to add this to the CSS to center the text lines:

.calendar_default_colheader_inner {
  text-align: center;
}