These docs are for an old release.
Info on upgrading to v5
weekNumbers
Determines if week numbers should be displayed on the calendar.
Boolean, default: false
If set to true, week numbers will be displayed in a separate left column in the Month/DayGrid views as well as at the top-left corner of the TimeGrid views. View a simple demo.
By default, FullCalendar will use the current locale’s week number calculation method. To display other types of week numbers, see weekNumberCalculation.