Chapter 121:6 & 7
Laws of Public Fast Days
6. If [the dates of] these public fasts fall on Shabbos, the fasts are
postponed until after Shabbos. If the tenth of Teves falls on Friday, the
fast is held on that day and completed.
7. When any of the four public fasts mentioned fall in the midst of the
seven days of rejoicing for a marriage, the groom is required to fast (1).
Although these days are considered like a festival ("regel") for him, this
"festival" is a personal celebration, and thus, the mourning and fasting of
the community as a whole take precedence. Also, [Psalms 137:6] states: "If
I forget you, O Jerusalem, let my right hand forget its skill. Let my
tongue adhere to my palate, if I fail to recall you, IF I FAIL TO ELEVATE
JERUSALEM ABOVE MY HIGHEST JOY" (2).
(1) Even if the date of these fasts falls on Shabbos, and the fast is
postponed until Sunday, a groom is still required to fast (Mogen Avrohom
(2) Since the public fasts recall the process which led to the destruction
of Jerusalem, they take precedence over the seven days of rejoicing for a