Mathematics of the Jewish Calendar/Anniversaries
A problem arises if an event first occurred in Adar, Adar Rishon or Adar Sheni, or on 30th of Cheshvan or Kislev, as these days do not occur every year.
For example, a boy becomes Bar Mitzvah (legally an adult male) on his 13th birthday. If a boy is born in Adar and 13 years later it is a leap year, his Bar Mitzvah is in Adar Sheni. If a boy is born in Adar Rishon or Adar Sheni and 13 years later it is a leap year, his Bar Mitzvah is in Adar Rishon or Adar Sheni as the case may be. However, if a boy is born in Adar Rishon or Adar Sheni and 13 years later it is not a leap year, his Bar Mitzvah is in Adar. Take two boys born in 5746, which was a leap year, on 16th Adar Rishon and 2nd Adar Sheni. Both were Bar Mitzvah in 5759, not a leap year. If in both cases the ceremony was on the Shabbat after their 13th birthdays, the older one had his on 18th Adar and the younger one on 4th Adar, two weeks earlier. However, if he was born on 30th Adar Rishon, and 13 years later it is not a leap year, he becomes Bar Mitzvah on 1st Nisan.
If a boy is born on 30th of Cheshvan or Kislev and 13 years later there is not a 30th of the month, he becomes Bar Mitzvah on 1st Kislev or Tevet respectively.
The same rules apply to when a girl becomes Bat Mitzvah (legally an adult female), which is on her 12th birthday (or 13th in reform Jews).
Concerning the observance of Yahrzeit (literally, "year time", the anniversary of the death of a close relative), there are various customs and a rabbi should be consulted.