This is how this system arrived at an agreement between the solar and the lunar periods: 8 solar years containing 2922 days, while 8 lunar years only contain 2832 days, there was a difference of 90 days, for which Cleostratus compensated by Intercalating 3 months of 30 days each, which were placed after the third, fifth and eighth year of the cycle.
"The Eleven Comedies"
Aristophanes et al
The old practice of Intercalating a month was of course dropped.
"Plutarch's Lives Volume III."
Plutarch
Hence it may be seen that by Intercalating a full month every three years, or which comes nearer to accuracy seven times in nineteen years, restores the coincidence of the solar and the lunar year, and consequently the moons to the same day of the month on which they fell nineteen years before.
"Our Calendar"
George Nichols Packer