The day of the Yahrzeit / יארצייט is the date of death according to Jewish calendar. There are rabbinical opinions, however, that if the burial was two or more days after the death, the first Yahrzeit is observed on the date of the burial while all subsequent Yahrzeits are observed on the date of death.
Sometimes it is not so trivial to determine the Jewish calendar date of the Yahrzeit, since the number of months per year varies, because Jewish leap years have an extra month — Adar II / אדר ב. Furthermore, the number of days in certain Jewish months (Cheshvan / חשון, Kislev / כסלו and Adar / אדר) also varies from year to year, just as the length of February in the civil calendar. These months contain either 29 or 30 days, depending on the year.
It is important to note that the Rosh Chodesh / ראש חודש - Head of the Month that follows a 30-day month is celebrated for two days: the last, thirtieth day of the previous month and the first day of the next month. The Rosh Chodesh that follows a 29-day month is celebrated for one day only — the first day of the next month.
Dates in Cheshvan and Kislev. If a death occurs on the thirtieth day of one of those months and the corresponding month of the next year has only 29 days, it is simply not possible to observe the Yahrzeit on the thirtieth day of the month.
One resolution of this problem is that the Yahrzeit is pushed to the next day, since the death indeed occurred on the day after the twenty-ninth day of the month and the day of the Yahrzeit is Rosh Chodesh just as the day of the death. The same rule can be applied to any subsequent year.
Many, however, have a custom to resolve the problem differently. They observe the first Yahrzeit on the twenty-ninth day of the month, because it is the last day of the month and the day of death was also on the last day of the month. According to this custom the Yahrzeit is observed on the twenty-ninth day of the month in every subsequent year, even when the month has 30 days.
Dates in Adar. The same problem occurs when the death was on the thirtieth day of the Jewish month of Adar I / אדר א in a leap year, which is the first day of the Rosh Chodesh of the following month of Adar II / אדר ב. In a regular year there is only one month of Adar / אדר and it contains only 29 days. Some rabbinical authorities apply the same reasoning as above and rule to push the Yahrzeit to the next day, which is Rosh Chodesh of the Jewish month of Nisan / ניסן. Others, however, rule that in a regular year the Yahrzeit should be observed on the first day of Rosh Chodesh Adar, i.e., the thirtieth day of the Jewish month of Shevat / שבט.
The rest of the days in the Jewish month of Adar / אדר also pose a problem. If the death was in Adar of a regular year, the prevailing custom among Sephardim is to observe the Yahrzeit in Adar II, while most Ashkenazim observe it in Adar I. Some are strict and observe the Yahrzeit in both Adar I and Adar II. However, if the death was in either Adar I or Adar II of a leap year, the Yahrzeit is observed only in the corresponding Adar during the following leap years and in the only Adar during regular years.
When exact date of death is not known. If one knows only approximately the date of death, one should select the last date in the range of possible dates. If the date of a parent’s death is not known at all, one should select a date that does not coincide with the other parent’s Yahrzeit, unless one has grounds to suspect that both parents died on the same day.
The prevailing custom is to determine the date of the Yahrzeit according to the time zone where the death occurred. If the death occurred after sunset, but before nightfall, one should ask a competent rabbi to determine the date of the Yahrzeit.