Now there are a few complications in trying to match a lunar calendar with the four seasons, which are determined by the sun. On the Hebrew calendar, a lunar month was from new moon to new moon, which is 29 days, 12 hours and 44/100ths of a minute. Twelve of these months equal 3541/4 days, which is approximately 11 days short of the solar year. If the Jews followed this calendar for 10 years, with a year of 354-1/4 days, soon their months would not correspond to the same seasons which they had formerly. Only a few more years and they would be celebrating their new year in the middle of the winter; and Abib instead of being “the spring or sprouting month” would be the month of rain and snow.
To overcome this problem, they made an adjustment by adding a thirteenth month to their year every few years, to bring the lunar year into conformity with the seasons. This practice is called intercalation. At first it seems the month was added wherever the ruling power dictated. However, a pattern was developed, and the additional month was added after the twelfth month in years 3,6,8,11,14,17 and 19 of a 19-year cycle. This brought them out approximately right with the solar year and the seasons.