Intercalation typically refers to the addition of the 13th month in the Jewish calendar (Adar II) used to make up the difference of 11 days between the solar and lunar years. A leap year occurs seven times in every cycle of 19 years (maḥazor ḥammah), namely in the years: 3, 6, 8, 11, 14, 17 and 19 of the cycle. The first Adar has 30 days, the second Adar 29. The number of days in a leap year is either 383, 384, or 385. The period between the first of Nisan and the first of Tishri is always 177 days. The intercalation of years was already practiced by the Sanhedrin in the Hasmonean and mishnaic periods. Among the factors then taken into consideration were the ripened state of the Omer ("barley") offered on Passover, and that of the bikkurim ("first fruits") sacrificed on Shavuot ("Leap Year")
Numerous calendrical texts were found at Qumran. The calendars at Qumran and that in Enoch (chs. 72-82) describe a 364-day calendar "in sharp contrast with the mainstream Jewish lunar calendar" (Stern, 235). The 364-day calendar always starts on the same day of the week and at Qumran this is explicitly identified as Wednesday (most festivals begin on a Wednesday).
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Bibliography[]
"Leap Year." Encyclopaedia Judaica. Ed. Michael Berenbaum and Fred Skolnik. 2nd ed. Vol. 12. Detroit: Macmillan Reference USA, 2007. 567. Gale Virtual Reference Library.
Stern, Sacha. "Qumran Calendars and Sectarianism." Pages 232-253 in The Oxford Handbook of the Dead Sea Scrolls. Eds. Timothy H. Lim and John J. Collins. Oxford: 2010.