Something like this…
https://www.dillards.com/p/gianni-bini-lori-off-the-shoulder-sheath-dress/507834248
My friends son is getting married next month. The moms are wearing long dresses and my friends is a floral print at a 5:30 indoor wedding, I don’t think there is anything wrong with floral in summer at all for guests, especially in the afternoon.
Black Tie Optional. 
@thumper1
But seriously, it’s a very small step below, except for the guys, where a tux would be out of place.
I think it’s fine, with no need to change if you don’t want to.
BB said it better than I did. When I mentioned sundress, I was thinking of the cotton ones that one would to the beach as a cover up. They have spaghetti straps and are short. Way too informal to wear to an office, let alone a wedding. The floral,dresses discussed in above posts are lovely
My silk sundress has spaghetti straps. Unless you come and touch it, it probably looks like cotton.
I’ve gotten past the point where I worry about these things. Really, at a wedding…no one is going to be looking at the guests clothing unless it’s REALLY out of whack (like jeans to an evening wedding). Everyone will be looking at the bride!
I’m with @thumper1 It’s unlikely anyone is grading the guests clothing choices, as long as they’re in the ballpark like she describes.
People won’t be looking at the bride for 5 hours. Unless I am really entertained at a wedding, I spend most of my time watching people and what they are wearing.
I think a lot of people either ignore or are blissfully unaware of the various meanings of dress codes these days. Things are definitely more varied now than a few decades ago, and I kinda think that’s a good thing.
Anyway, for me as a guy, here’s the ranking:
Semi-formal: suit and tie. Ladies, wear what you’d feel comfortable in going out to dinner where all the men are wearing suits and tables have white tablecloths with stuffy waiters and long wine lists.
Black tie optional: A more formal suit, or a tux for the men.
Black tie / formal: Tux required, think red carpet at the Oscars or prom for the kids.
White tie: A step higher, a formal ball held by the prince of some tiny European country to celebrate their centennial.
I’m not sure if there’s a formal term for a touch less casual than semi-formal, e.g. slacks and blazer ok and skip the tie. That’s sort of business-casual but dressed up a touch, like maybe “summer formal” or maybe “cocktail party dress”.
“I’m not sure if there’s a formal term for a touch less casual than semi-formal, e.g. slacks and blazer ok and skip the tie. That’s sort of business-casual but dressed up a touch, like maybe “summer formal” or maybe “cocktail party dress”.”
A club where we’re members have been struggling with this for years with somewhat comical results. Here in Florida, it’s already a mishmash of regional backgrounds so there really is no prevailing dress code and even at the Ritz, you’ll see everything from ball gowns to flip flops. Each year, the club house committee gets new members that are horrified by something they saw in the dining room and attempt to clarify the dress code. Some of the funnier phrases they’ve tried have been smart casual, dressy casual, relaxed formal, seasonal casual, etc. But since there’s no general consensus about what those phrases mean, you still have people show up in an assortment of gear.
I think the floral dress would be perfect and appropriate for the wedding described in the OP, especially since it’s outdoors at 3 PM. But then again, I’m pretty tolerant of a wide range of clothing and think it’s nearly impossible to dictate what people will look good in. Some people can carry off ripped jeans and flip flops yet still look like they’re ready for a Buckingham Palace event and others look sloppy and unappealing in their $10,000 formal gown. Dressing well is like that phrase about how art differs from pornography… tough to define but you know it when you see it.
Dressy casual. Although what that actually means is open to interpretation, IMO.
Slightly more complex than that. For men, it’s literally a white bow tie, along with black tailcoat, white vest, plus accessories.
You reminded me of another one, “Resort casual”. To me that denotes a well-dressed casual outfit. Shorts ok but should be a high quality (e.g. no board shorts or running shorts), short sleeve shirts ok but again a dressy short sleeve (for guys that would mean a polo shirt or a nice t-shirt, not your beat-up Nike running shirt). When my company hosts “reward” events for the sales team at resorts they usually specify “resort casual” for meetings.
Given how many threads pop up about what all the terms mean I’m beginning to think all invitations should come with detailed instructions. “No jeans, no shirts without collars, no need for a tie, no floral dresses, no long gowns, no flip flops unless they are bejeweled.” Our next door neighbors sent us an invitation to what we called their “Not-wedding”. Fancy, but not formal invitation, huge tent in their yard. Only a few older men wore ties or jackets. Most of the women wore black or white either very long or very short dresses. One guy wore pink and white seersucker shorts, but I think everyone else had long pants. We actually peaked out the door to see what the guests were wearing before we got dressed! The “not-bride” wore a long white dress with revealing cutouts. The “not-groom” had a polo shirt. They cut a cake with two tiers, but no vows were said. There was a toast, but no formal engagement announcement. (Others at the party thought it was an engagement party. Whatever it was, we had a good time!)
Those sound like awesome, fun neighbors, @mathmom ! My grandfather was an artist, a dancer and very fun person and always had fabulous friends. But my absolute favorites were the funny friends he met when they threw a “Celebration Funeral” and randomly invited people they didn’t know in the neighborhood. They left odd, cryptic but funny invites in everyone’s mailbox. From the invite, it was tough to understand if it was a birthday party or funeral but the final line gave a broad hint… “The deceased will make every effort to attend.” Of course, my grandfather couldn’t resist this and his curiosity was rewarded by a great party and great new friends.
D1 had a little blurb about dress code for each event on her wedding website. It took guess work out for people and made it less stressful.
I give up! I’ve tried to write this response three times and when I hit “save comment” it deletes it.
My message…if you aren’t sure what to wear, ask the MOB or MOG…or other friends attending.
I like that, oldfort. This being CC, I expect someone to pop in and say that it is tacky to tell the guests what to wear… 
Ha! Bejeweled flip flops! My D wore them as a bride! With her tea length gown. (beachside wedding).
Her gown was lovely, her H wore a tux, and the invitations said–wear whatever you want.
Best. Wedding. Ever. Five years later, I’m still sure about that.
If ANYONE clutched their pearls, or instagrammed in horror–their loss.
My relative wore rhinestone studded flipflops under her gorgeous floor-length gown and was very happy and comfortable. No one batted an eye and she had a wonderful time!
From Bride’s magazine, dress codes defined: https://www.brides.com/story/wedding-dress-code-explained
That being said, I’m from the South where a “formal” 3 p.m. wedding means morning wear, not black tie (like the princes: https://www.countrylife.co.uk/luxury/style/morning-dress-weddings-155093 )
If the floral dress is a nice silk, I think it would be fine for a 3 p.m. outdoor wedding.
One of my favorite things at my daughter’s wedding this past weekend was looking at the guests’ attire. It was a morning wedding with a brunch reception immediately following, and I think my daughter specified on her website that the attire was “garden party.” Most of the women came in floral dresses. A few wore hats. The men were all over the map, from suits to navy blue blazers and khaki pants, to seersucker suits to nice, collared shirts and khakis.