This year, for the first time since 5762 (2001), the Fast of Tevet will be observed on a Friday. This creates an awkward situation, where one is preparing for Shabbat while also fasting. In perhaps the greatest contrast, at erev Shabbat mincha, one reads Torah and Haftarah as normal on a fast day, before going into Kabbalat Shabbat and Maariv and then breaking the fast at Shabbat Kiddush.i
This may not be an enormous undertaking in the Northern Hemisphere, when Shabbat in late December or early January starts mid afternoon, but in Buenos Aires or Cape Town or Melbourne this could be a significant hardship. Still, the fast is observed on a Friday worldwide.
The situation raises three questions:
1) What are the calendar mechanics that cause the 10th of Tevet to fall on a Friday?
2) What would happen if another fast day fell on a Friday?
3) Why is the Tenth of Tevet so special?
First, a brief primer on how the Jewish calendar is calculated. It consists of lunar months, which are between 29 and 30 days, over a fixed 19-year cycle of non-leap (12-month) and leap (13-month) years. This is necessary because a non-leap lunar year is approximately 11 days short when compared to a solar year and so must be augmented to keep the holidays seasonal. (This is in contrast to the Muslim calendar, which is purely lunar with no leap years and so the holidays shift throughout the year.)
There are also constraints such as on when the first day of Rosh Hashana can be (with the mnemonic לא אד"ו ראש – meaning not on Sunday, Wednesday, or Friday). This is so that Yom Kippur does not fall on Friday or Sunday and so that Hoshana Rabbah does not fall on Saturday.
These constraints result in at times needing to add one or two days to each year (both leap and non-leap) to make the next RH come out right. Days are added to Cheshvan (and potentially Kislev), which always makes them fall between Simchat Torah and the end of Chanukah. As a result, the period from Purim to ST always has the same number of days and so there is a 1-1 relationship between the days of the week of RH and most other holidays.ii
Based on this calendar algorithm, for the next 100 years (2010-2109), the 10th of Tevet will fall on a Friday 21% of the time.
10th of Tevet
|Day of the week
While this hasn't happened since 2001, 1996, 1993, and 1983, it will happen again in 2013, then 2020, 2023, and 2024 (which actually occurs in Jan 2025).
The only other fast that could occur on a Friday would be Ta'anit Bechorot (because of a Saturday Pesach and therefore a Monday Rosh Hashana). If this happens, though, it is pulled up to the previous Thursday instead of observed on a Friday.iv
So why is the 10th of Tevet so special that we observe it on a Friday? And why do we ensure in the calendar that it cannot fall on Shabbat? It seems that there is a biblical relationship between the 10th of Tevet and the other fast day that we structure the calendar around instead of shifting its observance: Yom Kippur. In Leviticus 23:28, Yom Kippur is described as בעצם היום הזה ("on the very day"). Similarly, in Ezekiel 24:2, 10th of Tevet is described using very similar language, as עצם היום הזה ("the very day").
This suggests that were the 10th of Tevet to fall on Shabbat (which is currently impossible) we'd actually fast, which would create a strange situation since it is not a fast day with anywhere near the theological significance of Yom Kippur. However, this is not an issue with a Friday that has no particular significance, and so the 10th of Tevet can fall on a Friday, with only minor inconveniences to Shabbat cooking and erev Shabbat mincha.
May we all have a צוֹם קַל – an easy and meaningful fast.
i Despite reading Torah and Haftarah, because it is erev Shabbat, one omits Tachanun as usual, and also Avinu Malkeinu. One still does bathe as usual in anticipation of Shabbat. See Shulchan Aruch, Orach Chaim 566.
ii There is a spectacular את בש mnemonic (formed by starting at either end of the alphabet and then pairing letters going inward from each end) for the days of the week that each holiday falls on, corresponding to the days of Pesach. It does not take into account holiday observance that is pushed off because of Shabbat. Say the first day of Pesach was a Tuesday. Then את means Tisha Bav is on a Tuesday, בש means Shavuot is a Wednesday, גר means that Rosh Hashanah is on a Thursday, דק means Kriat Torah – reading of the Torah - Simchat Torah is on a Friday, הצ means Tzom – fast – Yom Kippur is on a Saturday, ופ for Purim on a Sunday, זע for Atzmaut – Israeli Independence Day on a Monday. However, (northern hemisphere) fall holidays are more flexible – Chanukah, 10th of Tevet, Tu Bishvat, due to the extra days in Cheshvan or Kislev.
The את בש goes back at least to the days of the Tur (1269-1340), though obviously there was no Yom Ha’Atzmaut back then. That its missing seventh day is now accounted for is an amazing/divine coincidence.
iii The fact that the 10th of Tevet can't fall on Monday either is interesting, though not crazy - since there are only three numbers of days it can be after RH, which can only be on four days of the week, but not every combination is possible - 353 only occurs with Monday and Saturday, 354 with Tuesday and Thursday, 355 and 383 and 385 with Monday, Thursday, and Saturday, 384 only with Tuesday. You'd need a 354 or 384 to occur with a Monday or Saturday RH, or 353 to occur with a Tuesday to get it on Monday or Saturday, none of which ever happens, since they would result in a Sunday or Friday RH the following year.
iv Several other fasts, if they fall on Shabbat, are observed a day late or two days early. In these cases (even for Tisha B’Av) certain special individuals can eat at very festive occasions that cannot be moved, such as those intimately involved with a bris, since the fast does not actually “fall” on that day but is merely “observed”.
If Tzom Gedaliah were to fall on Saturday (because of a Thursday RH) it is pushed off to Sunday (it cannot fall on a Friday because that would mean a Wednesday RH)
If Ta'anit Esther were to fall on a Saturday (because of a Sunday Purim and therefore a Thursday RH) it is pulled up to the previous Thursday (it cannot fall on a Friday because that would mean Purim on Shabbat which would mean a Wednesday RH)
If Ta'anit Bechorot were to fall on a Saturday (because of a Sunday Pesach and there a Tuesday RH) it is pulled up to the previous Thursday.
If 17th of Tammuz / Tisha B’Av were to fall on a Saturday (because of a Monday RH) it is pushed off to Sunday (it cannot fall on a Friday because that would mean a Sunday RH).
Technically, the beginning of Tisha B’Av can overlap with the end of Shabbat, either because it actually falls on Sunday or is pushed off from Saturday. But one does not experience fasting during this hour, since one has just finished the pre-fast meal.