There are questions that had solid answers in the 1950s, that no longer are quite so clear in this day and age, and attire for a Bridal Shower is one of those, because the possible variations in theme, setting, and tone encompass such a wide range.
Is the shower going to be with the Bride-to-be’s girlfriends? Perhaps running out to something fabulous or crazy or even naughty? This would differ from one with the Bride-to-be’s coworkers, who might be getting together for a small after-work celebration. And it would certainly be different from one with sisters, cousins, aunts, and other family, and friends-of-family, guests.
Since the first two options here are what they are, and the participants no doubt are setting the tone as plans are made, the main question for “what to wear” to a Bridal Shower is likely to focus on the latter … the one with all the family pressure.
One phrase that comes up in discussing Bridal Shower outfits repeatedly is “grandmother ready” … in that you don’t want to be showing up in something that is likely to garner the disapproval of the Bride-to-be’s grandmother. Needless to say, this trims down the options. Another “rule of thumb” is only the Bride gets to wear white (and she should be wearing white), and that near-white (cream, ivory, etc.) should probably be avoided as well. Oddly, black is frowned upon, as well as red … perhaps to not be suggestive of either death or harlotry.
Also, with this being a most feminine of events, dresses are certainly preferred over pants suits or “business casual” … unless this is one of the rare Bridal Showers where the guys are invited as well, as they would do well to wear business casual, and generally stay out of the way.
The timing, date, and location of the Bridal Shower should certainly be taken into account, as what would be ideal for a May event might not work so well at something feting winter nuptials, and one might well wear one thing to the Bride-to-be’s home and another outfit if the event is at a restaurant or other function space.
As always, rules get over-ruled by specific requests from the hostess, so if the Bride-to-be’s family says “luau theme”, you’re not likely to want to show up dressed for a networking event.
The Bridal Shower, of course, is not a date, so while one is dressing up for a special occasion, one should avoid trying to be the focus of attention. This is a situation where comfort can trump style, to an extent, so flowing dresses, and merciful shoes are called for, but within the context of being “dressed up” (unless being part of a party theme, jeans, sweats, and other “casual casual” clothes are not appropriate).
Again, if you’re the Bride-to-be, find something fun and comfortable in white, but for everybody else, you’re there to celebrate her upcoming day, and the focus should be on making the event as much of an enjoyable party as possible for her.