The Metonic Cycle
This chart plots the months of the Hebrew calendar against the months of the civil calendar for the civil years 2016–2035. Rosh HaShanah is the first day of Tishrei (shown in blue).
A leap month (shown in yellow) is added to the calendar 7 times every 19 years. This is the Metonic cycle, where 235 lunations (19 × 12 + 7) is almost exactly equal to 19 years. The Hebrew Year 5777 starts Metonic cycle #305, and 5796 starts Metonic cycle #306.
The month of Cheshvan may be lengthened from 29 to 30 days (indicated by +), and the month of Kislev may be shortened from 30 to 29 days (indicated by −). These adjustments are made to prevent Rosh HaShanah from falling on Sunday, Wednesday, or Friday.