There was once a 13 MONTH calendar adhered to by MANY 'ancient cultures' based on the 13 MOON cycles in a year, with each cycle lasting approximately 28 days. (13×28=364 +1... marking the RISING of the 'new sun')

The names of MODERN months reflect 'remnants', of an OLDER system:
SEPTEMber is derived from 7 but is now the 9th month, OCTOber, means 8, is now the 10th month, NOVEMber, 9, is now the 11th and DECember, 10, now the 12th.