I am confused about the halacha of wearing clean laundry, I did not grow up with this custom of putting on all clean clothing before the 9 days, but I do hear that people do it… is it a minhag or halacha? if it’s an halacha what do you do if you didn’t pre-wear it?
There is no halacha that you have to put clothes on before the nine days. It is done from a practical standpoint. This is since the custom among the Ashkenazim is not to wear freshly laundered clothing during the nine days from Rosh Chodesh Av until after Tisha B’av, and for Sefardim from Shabbat Chazon (This is brought in Shulchan Aruch O:CH 551), therefore people wear their laundered clothing before the nine days in order that they should be able to wear them.
If you didn’t wear your clothes before the nine days there are other option given by the poskim in order to take the “freshly laundered feel” out of the clothes.
According to some poskim, if the clothing were put on the floor for about an hour, it will no longer be considered freshly laundered. Another option is to put the clean clothes into the hamper with the unclean clothes, and it will get a little of the smell of the unclean clothing, this will also render it not freshly laundered.
A third option is based on the ruling that on Shabbos we are allowed to wear clean clothing, therefore one can put the clean clothes on during shabbos and wear them for a little while, then after shabbos it may be worn because it is no longer fresh.
Minchas Yitzchok 10-44, Halichos Vhanhagos pg. 8 in the name of R’ Eliyashiv zt”l, Orchos Rabeinu 2 pg. 130.