These docs are for an old release.
Info on upgrading to v4
columnFormat
Determines the text that will be displayed on the calendar’s column headings.
This settings has been renamed to columnHeaderFormat.
format string, default:
'ddd' // like 'Mon', for month view
'ddd M/D' // like 'Mon 9/7', for week views
'dddd' // like 'Monday', for day views
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.
The default values will change based on the current locale.