Years ago I worked for a Catholic newspaper, and we did a sort of “Reader Poll” on how people should dress for Mass. That got a HUGE response and it seemed like everybody had an opinion of some kind.
The number one concern (rightly so in my mind) was simple modesty – women/girls dressing in a way that didn’t become an occasion of sin to the men present, and girls AND boys not wearing shirts, jackets, caps, etc. with potentially offensive designs or slogans on them (rock band logos, advertisements for liquor or bars, gang symbols, etc.) This, I believe, is and should remain the number one priority.
Farther down the list was the question of whether you should always wear “dress” clothes for Mass or if casual attire such as jeans, plain T-shirts or polo shirts, sweatshirts, etc. was OK. Most said “business casual” type attire was OK, and jeans were OK as long as they were clean and intact. Some thought shorts should never be worn to Mass, others said shorts of decent length were OK. **Several people objected to flip-flops on the grounds that they make a distracting “flip floppy” noise when the person wearing them walks up to Communion (which is quite true, and I have avoided wearing them ever since for that reason).
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My personal rule is to dress as I would for work (I work in a state government office with a business casual dress code). My daughter attends a public school with a uniform-type dress code that permits polo or button-down shirts in specified colors, shorts that are no more than 6" above the crease at the back of the knee, and black, navy blue or khaki slacks or skirts with belts (no denim). So I just have her wear her school clothes to Mass, or a nice dress if she chooses to wear one.
Now some people argue that you should always dress for Mass as you would if you were going to see the pope, or the Queen of England, or the POTUS, or some other earthly dignitary. Although that’s a nice sentiment, I don’t think it’s practical for most people – because for most people, going to see the pope, queen, president, etc. is a once in a lifetime event, while Sunday Mass isn’t. So I don’t think everyone is obliged to wear a suit and tie, or a skirt suit/dress with high heels and pantyhose, to EVERY Sunday Mass, unless they want to.