The History of Time Leofranc Holford Strevens Oxford University Press 2005 p 101 has the following rule A 13th month is added under current rules in years 3 6 9 11 14 17 and 19 of a Metonic cycle nine years behind the Western Golden Number cycle the month repeated which must not be a winter month 1 11 or 12 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 2 5 8 and 11 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 86 173 166 100