1,379. Yahrtzeit on a Two-Day Rosh Chodesh
221:4 The month of Cheshvan is sometimes 30 days long ("full"), in which case Rosh Chodesh Kislev, which comes after it, is two days. The first day of Rosh Chodesh is the 30th day of Cheshvan and the second is the first day of Kislev. This is the case with every two-day Rosh Chodesh: the first day is the 30th day of the previous month. Sometimes Cheshvan only has 29 days ("deficient") and Rosh Chodesh Kislev is only one day. Similarly, the month of Kislev is sometimes full, with the result that Rosh Chodesh Teves, which comes after it, is two days. Sometimes it is deficient, so Rosh Chodesh Teves is only one day.
If a person died on a Rosh Chodesh Kislev that was only one day, then in a year when Rosh Chodesh Kislev is two days the yahrtzeit would be the second day, which is the first day of the month of Kislev. This is because the deceased passed away on the first day of Kislev. If he died on the first day of a Rosh Chodesh Kislev that was two days, then in a year when Rosh Chodesh is only one day, there is a doubt whether to observe the yahrtzeit on the 29th of Cheshvan - since the deceased passed away at the end of Cheshvan - or, since vows follow the way people speak, on "Rosh Chodesh Kislev," which is what people call it.
One should act as follows: if Rosh Chodesh Kislev in the first year coming is only one day, he should observe the yahrtzeit on 29 Cheshvan, and do similarly whenever Cheshvan is deficient. On the day after Rosh Chodesh, if there are no mourners, he should recite kaddish and lead the prayer service. He should only do this if he would not be taking it away from someone else. However, if Rosh Chodesh Kislev in the first year coming is two days, he should observe the yahrtzeit on the first day of Rosh Chodesh. He should then always observe this date, even when Rosh Chodesh is only one day. The same is true when it comes to Rosh Chodesh Teves.
221:5 Sometimes Rosh Chodesh Teves is only one day, namely the sixth day of Chanukah. This makes the last three days of Chanukah the first, second and third of Teves. Other times, however, Rosh Chodesh Teves is two days: the sixth and seventh days of Chanukah. In that case, the seventh day of Chanukah is 1 Teves and the eighth day is 2 Teves. Therefore, a person who has yahrtzeit at this time must be careful to count according to the date and not by the days of Chanukah.