Version 7 will be released soon.
Read the v7 docs
multiMonthTitleFormat 6.1.0
In Multi-Month views, the format of the text above each month.
If the calendar spans a single year, the default is { month: 'long' } (ex: "January").
If the calendar spans multiple years, the default is { month: 'long', year: 'numeric' } (ex: "January 2023").