The History of Time Leofranc Holford  Strevens Oxford University Press  p has the following rule  A th month is added under current rules in years       and  of a Metonic cycle nine years behind the Western Golden Number cycle the month repeated which must not be a winter month   or  is that throughout which the sun will remain within the same sign of the zodiac subject to the rule that the equinoxes and solstices must fall in months    and  if this would be breached intercalation is postponed till after the second month of the following year  I have not found Holford  Strevens to be a particularly reliable source