columnHeaderFormat

Determines the text that will be displayed on the calendar’s column headings.

// like 'Mon', for month view
{ weekday: 'short' }

// like 'Mon 9/7', for week views
{ weekday: 'short', month: 'numeric', day: 'numeric', omitCommas: true }

// like 'Monday', for day views
{ weekday: 'long' }

As noted above, each view has a specific default. Get fine-tuned control with View-Specific Options. A single string alone will set the value for all views.

If you need programmatic control, use columnHeaderText or columnHeaderHtml instead.