How to know if Netzavim and Vayeilech are read together or separately
Note: This is taken from a Facebook post on 8/31/21
I was going to post this later in the week, but I was asked today if Netzavim and Vayeilich are read together or separately this year (spoiler for those who don't want to read more: Separately). Unlike most other double parshas which are either mostly or fully dependent on whether or not there is a leap year, this one is determined solely based on when Rosh Hashanah falls out (with some fun quirks as a result).
As mentioned in other posts, Devarim always falls out the Shabbos before Tisha B'av. Therefore, there are 9 parshas that need to be read between the day after Tisha B'av and the day before Sukkos - Va'etchanan/Eikev/Re'eh/Shoftim/Ki Teitzei/Ki Tavo/Netzavim/Vayeilech/Ha'azinu. Av has 30 days while Elul has 29, meaning there is a 21+29+14 = 64 day window to read these parshas. Given that 63 days is exactly 9 weeks, there are exactly 9 Shabboses in this time span unless both day 1 and day 64 are both Shabbos, which thankfully is an impossible scenario (Sukkos can't begin on Sunday).
So...9 Shabboses to read 9 Parshas. Simple enough, right? Obviously not (because it's assur for anything to be simple). But this should make the picture clearer: If you can read a parsha on each Shabbos, every parsha is read on its own. However, if there is one where you can't, two parshas will need to be combined.
The latter scenario occurs when Rosh Hashanah is on Thursday (which results in Yom Kippur on Shabbos) or Shabbos. In those years (most recently this past RH), Netzavim and Vayeilech are combined. In other years (RH on Monday/Tuesday, but would apply if it could fall out on other days), like this coming year and the next, they are read separately.
A few resulting quirks:
1. Even though it's not the cause, the effect of this is that Monday/Tuesday RH result in a Shabbos between Yom Kippur and Sukkos, so you can look at the whole thing in reverse.
2. The above results in Shabbos Shuva being Vayeilech on those years and Haazinu when RH is on Thursday or Shabbos. Either way, the haftorah read is the one generally listed for Vayeilech despite it being more likely to be Haazinu (due to Tuesday RH being half as likely as the others).
3. My favorite of all, this all results in Vayeilech either being the last Shabbos of a given Hebrew year (when combined with Netzavim) or the first (if read alone). That means the parsha can be read between 0 and 2 times in a single year. Vayeilech was read with Netzavim to end 5780 and is read alone next week to begin 5782, meaning it is not read at all in 5781. In 2 years (5783, where RH is on Monday and the following RH is on Shabbos), Vayeilech will be read twice.
Lastly, the impossible scenario mentioned earlier would result in 10 Shabboses to read 9 parshas. If it were allowed, I wonder what we would do. If anyone has an idea, please let me know.