I’m going to a Catholic church for the first time. What should I wear?
A normal city parish on a Sunday? Slacks, collared shirt, a tie if you are going with someone who wears a tie. A dress or a skirt or slacks with a blouse. Add a coat if it is cold and bring one or a sweater in case they have the air conditioning set at arctic!
There is no dress regulation for catholic women, the only place where some modesty is expected is at church, usually meaning covered shoulders and knees (which means pants are perfectly acceptable for women). ... Other than that they can wear pants, cleavage, miniskirts or whatever they want.
This is especially true with the Major basilicas. Shirts without sleeves are not permitted, men may not wear shorts, and women's skirts must reach to below the knees. Upon meeting the Pope, or taking part in Papal ceremonies, the preferred mode of dress is either a business suit or in national costume.