These docs are for an old release. Info on upgrading to version 2/3
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monthNames 1.3

Full names of months.

Array, default:

['January', 'February', 'March', 'April', 'May', 'June', 'July',
  'August', 'September', 'October', 'November', 'December']

Prior to version 1.3, this was possible by setting $.fullCalendar.monthNames