Why Does The Hebrews Calendar Need Periodic Adjustment?
The very early Springtime Feast / Festival of Firstfruits is the key! The calendar is adjusted because a solar year is 365 days long while a lunar year is only 354 days, 11 days shorter. In order to catch-up to the solar year an extra month is added in certain years, this is a Hebrew leap year. The 12th (last) Hebrew month is called Adar, but in a Hebrew leap year a 13th month is added called Adar Sheni (the second Adar).