Question:
May wife wants to buy herself new nightwear to be worn only for mitzvas onah … may it be purchased in the nine days?
Answer:
She should wait until after Tisha B’av to buy it. Although she will only wear it when she means it for mitzva, nevertheless it is a garment that is being worn, and the heter of dvar mitzva is essentially only by tefillin which is an item that has no other use but mitzva. Another example of this would be that one can not buy a new suit during the nine days even though he will only wear it on Shabbos.
Sources:
Igros Moshe O:CH 3-80
