These docs are for an old release.
Info on upgrading to v5
titleFormat
Determines the text that will be displayed in the header’s title.
Date Formatter, default:
{ year: 'numeric', month: 'long' } // like 'September 2009', for month view
{ year: 'numeric', month: 'short', day: 'numeric' } // like 'Sep 13 2009', for week views
{ year: 'numeric', month: 'long', day: 'numeric' } // like 'September 8 2009', 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.
For views that display date-range text, to control the text between the two dates, customize titleRangeSeparator.