Greek 202 / 302 Aristophanes The Athenian Calendar

The Athenian year has twelve lunar months (alternating 29 and 30 days)
giving a total of 354 days (12 x 29.5).

It begins on the first new moon after the summer solstice (21st June).

Three times during an eight year-period, a second Poseideon is inserted into the calendar.
(365.25 days - 354 days = 11.25 days x 8 years = 90 days = 3 extra months)