211:10 A mourner may not wear his Shabbos clothes during shloshim, not even on Shabbos; he certainly may not wear new clothes. When mourning a parent, the practice is to prohibit new clothes for the entire twelve-month mourning period. However, if the mourner needs them, the clothes should be given to another person to wear first for two or three days
211:11 Let’s say that a woman is in shloshim, or even in shiva, and it’s time for her to go to shul on Shabbos after having a baby. The practice is for her to treat this Shabbos like a festival, wearing fancy clothes and jewelry. In such a case, the woman in mourning is permitted to wear her normal Shabbos clothes but not her special yom tov clothes so that she should not come to forget her state of mourning. She need not sit in a different place in shul (compare 211:15).