Upshernish During The Three Weeks
If a boy is born between 17 Tammuz and Tisha B’Av, when should the Upshernish be made?
If it must be done after Tisha B’Av, must it be done immediately, i.e. the following day?
When Tisha B’av falls on Sunday or Tuesday: on 10 Av after Chatzos (of the day).
When it falls on Thursday: on Friday, 10 Av, even before Chatzos.
When it falls on Shabbos: on Sunday night.
See also here:
It should not be delayed past this day. Generally, we don’t delay upsherenishes, particularly in this case, where it was already delayed so much.
The Rebbe writes that even when the actual Upshernish takes place after the child’s third birthday, the custom to start educating him in wearing Tzitzis, Birkas Hashachar, Bentching and Krias Shma before bed, start on the day of his third birthday.