The 2 year Rosh Chodesh

Which is also Chanukah (obviously)

As many of you may know, Rosh Chodesh Tevet is tomorrow (technically starts tonight) and Wednesday. Those dates happen to also be December 31^st and January 1^st, meaning Rosh Chodesh Tevet spans across 2 years.

Before dealing with the scenarios, let’s distinguish this phenomenon from the Thanksgiving one where Rosh Chodesh falls out on both that day and the next. Thanksgiving is tied to a specific day of the week but a range of calendar dates, while New Year’s eve/day are tied to specific calendar dates but not a specific day of the week.

For Rosh Chodesh Tevet to be 2 days in general, Kislev must have 30 days. While it technically doesn’t matter how many days Cheshvan has, not every year type allows a 30 day Kislev with both a 29 or 30 day Cheshvan.

Doing some quick math, 30 Kislev is 88 days after Rosh Hashanah if Cheshvan is 29 days and 89 days after Rosh Hashanah if Cheshvan is 30 days. That means that Rosh Hashanah must fall out on either October 3^rd or 4^th for Rosh Chodesh Tevet to fall out across 2 years. As those are very late dates for Rosh Hashanah (as mentioned before, this year in the cycle tends to have the latest dates), this can only happen on years following leap years and not on a leap year (until the calendar drifts later, as also mentioned before). That leaves the following scenarios:

1. Monday Rosh Hashanah on October 3^rd, 30 day Cheshvan/Kislev

2. Tuesday Rosh Hashanah on October 4^th, 29 day Cheshvan/30 day Kislev

3. Thursday Rosh Hashanah on October 3^rd, 30 day Cheshvan/Kislev (what we have this year)

4. Thursday Rosh Hashanah on October 4^th, 29 day Cheshvan/30 day Kislev

5. Shabbos Rosh Hashanah on October 3^rd, 30 day Cheshvan/Kislev

Prior to this year, we last had this phenomenon in 2005-2006 (scenario 2 above) and before that in 1959-1960 (scenario 5). It will happen next in 2035-2036 (scenario 4) and 2054-2055 (scenario 5). Note that the next 2 occurrences are 11 and 30 years from now, both year 1 of a 19 year cycle and not year 9 like this year. Also, the above occurrences contained each scenario except for scenario 1, so to not leave it out let’s note that it happened in 1921-1922.

Lastly, there have been a few close calls. Most recently, Rosh Hashanah 2016 fell out on October 3^rd but Kislev only had 29 days. Before that, Rosh Hashanah 1986 fell out on October 4^th but Cheshvan also had 30 days that year. As a result, Rosh Chodesh Tevet fell out on January 1^st and 2^nd, 1987.

As always, let me know if I got anything wrong. Happy Rosh Chodesh and Chanukah everyone!