Jewish Leap Years Explained: What They Mean and Why They Matter

Adding an extra month to the year sounds pretty simple. Instead of one month of Adar, there are two. It really doesn't sound that complicated.

And yet, this situation can actually prompt some interesting halachic questions. For example, a person born in the month of Adar I (in a leap year) is born in a unique month that doesn't exist every year. What should this person do in a non-leap year? When is his birthday? And, when he turns 13, when is his Bar Mitzvah?

Some might be inclined to dismiss this question and say that he should celebrate his Bar Mitzvah in Adar I. 

But imagine the following scenario: A child is born on the 20th of Adar I, and his friend is born on the 10th of Adar II. By all accounts, the first child is 20 days older than the second.

13 years later isn't a leap year. The first child will celebrate his Bar Mitzvah on the 20th of Adar, while the second child, who is 20 days younger, will celebrate 10 days earlier, on the 10th of Adar.

Accordingly, the child who is 20 days younger than his friend could be considered an adult according to Torah law, while the older one would still be considered a minor.

When the 13th year is a leap year, most halachic authorities rule that even someone born in Adar II becomes 13 in Adar I, since 13 years have passed since his birth.

What about a yahrtzeit (the anniversary of someone's death)?

Halachic authorities are divided on this issue.

If a person's parent passed away in the month of Adar and he wants to visit the grave or fast on the yahrzeit, according to the Shulchan Aruch, he should do so in Adar II, and according to the Rema, he should do so in Adar I.

Regarding laws determined by the passing of a "year" - for example, if someone makes a vow for the duration of a year - the vow includes the additional month of Adar, since a leap year is by definition a year with thirteen months.