These docs are for an old release.
Info on upgrading to v6
dayHeaderFormat
Determines the text that will be displayed on the calendar’s column headings.
Date Formatter, default:
// 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 the day-header render hooks instead.