The black tie optional is a clue to dress up on the guys; I agree with the comments about women being dressy but with out of doors making sure about footwear on grass or other outdoor surfaces, and if the day has any wind…nice to know. To me, better a little over dressed than under dressed.
I would think this calls for something soft and long…more like a full-length, dressier sundress! My S’s wedding will be cocktail attire at 6pm. They don’t want black tie, but they do expect suits for the men. I suspect most women will wear short, LBD-type attire. A lot may depend on the weather - it could be very hot and humid or it could be cooler.
“O.K., CC wedding experts: What the heck is “Garden Party Formal - Black Tie Optional attire”???”
It is a pain in the neck, that’s what it is. YMMV. I’m with thumper in hoping to omit requirements. People know the venue and that it is a wedding. They will dress appropriately or perhaps not. Apart from site specific info (ie: suggest flats or wedges as we will be married in a rustic meadow and will be dressed casually), I prefer to omit instructions. I have had a bride hosting a 1920’s wedding (but it was really a stage production) tell me to dress like Lady Mary, Downton Abbey. Eh, I’m almost old enough to be her grandmother and lack a costume department, but sure… She seemed to view the guests as extras on set.
ITA with @travelnut. A real pain indeed. If I were invited to a wedding outdoors, I’d appreciate a warning about footwear or weather extremes. Otherwise, I think it’s rather insulting to guests to treat them as props in a big production.
We expect to see a wide range of clothing at D1’s wedding. Some men will wear dark suits, white shirts, and ties. Others may come in plaid shirts and khakis. Most women will wear cocktail dresses, but there will likely be a few who wear anything from denim skirts to casual maxi dresses. It’s not unusual (around here at least) to see a woman wearing a gorgeous cocktail dress, killer heels and jewelry with a husband who’s wearing Dockers, a sports coat, open neck oxford cloth shirt, and loafers. I don’t care as long as we all enjoy ourselves.
The only comment the bride-to-be has made is that she hopes her future MiL won’t wear black, and assumes she wouldn’t wear white or ivory.
@gosmom- what time of day is the wedding?
Travelnut, the Ralph Lauren store near me had a dress that is knee length, with separate black sheath, then lace o relay dress. It was so pretty, my g/f and I each bought the dress. On sale from $600+ to $200. After a year, I will, be wearing mine soon. We joke it is our Abbey dress.
It’s a 6pm wedding…in a rustic vineyard/ranch setting…so to some extent, the garden party/black tie optional is a bit befuddling to me. But on the footwear aspect, I know I won’t be wearing stilletto’s…
@gosmom - understand your confusion. A cursory google search suggests that pairing of dress codes may be unique. I think you’ll be fine dressed up without stilettos. Ambiguity may be your friend here.
For the men, ‘black tie optional’ - it does depend on the crowd too. I attend an annual fundraiser that is ‘black tie optional’ but all the older guys own tuxes and they wear them. The women, for the most part, wear dressed up - a few wear beaded/full length gowns but most are just dressed up nicely.
When someone puts “black tie optional” it makes me think it’s an option for some of the men to wear tuxedos…which is their dress wear for a black tie event.
My opinion…if it’s not “black tie” putting “black tie optional” makes no sense.
I mean really…do they expect anyone but the bridal party to show up in a tuxedo (and lots of bridal party guys aren’t wearing them either anymore).
I agree it’s confusing. The Knot (not my go-to, but easy) says
“A tuxedo isn’t required, but the wedding is formal enough for one to be appropriate. Go with a dark suit if it’s a nighttime wedding, regardless.
Him: If not a tuxedo, a dark suit and tie.
Her: A long dress, a really dressy suit, or something cocktail-length (but still formal). Glamorous makeup and dramatic jewelry and hair are appropriate.”
But “Garden Party” hints at flowery, summer, daytime. I guess adding "Formal " means rather dressy floral, not light material. Maybe lacey, in a light-ish color?
Not like most of us have this hanging in our closets.
Gosh – in my closet, I have “Garden Party” formal, dressy, everyday and casual. I also have “Rustic Chic” formal, dressy, everyday and casual. I also have “Black Tie Optional” formal, dressy, everyday and casual. I also have “Black Tie” formal, dressy, everyday and casual. I also have formal, dressy, everyday and casual for weddings where no specific dress code is indicated at all.
And don’t get me started on the numerous outfits I have for Bar Mitzvahs – Orthodox formal, Conservative formal, Reform formal, Reconstructionist formal, and so on and so forth.
Doesn’t everyone? :))
“What the heck is “Garden Party Formal - Black Tie Optional attire”???”
What does it mean or what do they intend for it to mean? In reality it’s a meaningless contradiction in terms. Either something is formal or it isn’t. An outdoor garden party really doesn’t meet the definition of formal. And black tie always allows “optional.”
In reality they want men to dress in dark suits or tuxedos, and women in long or dressy cocktail dresses that are lightweight and flowy. In other words nothing heavy, no non-breathable fabrics like satin or overembellished gowns with heavy, hot jewelry. And for your own comfort and preservation of your shoes, you might want to think about wearing wedges or block heels rather than tall stilettos.
But I think most people could have managed equally well without a dress code.
Formal garden party attire: what one would wear to Ascot. 
https://www.ascot.co.uk/dresscode/flat-season#flatsseasoncarousel1
Seriously, to me, avoid metallics or sequins or blingy evening wear that would scream urban evening wear. Shoes that enable you to walk on grass. I’m assuming this is an afternoon wedding?
The perfect shoes! Not that I would ever actually spend that much on shoes, but I love them.
ETA, I spent so much time playing on the Bergdorf’s site that I missed the 6pm start date.
Iffy and possibly muddy ground would discourage me from wearing suede shoes, as those could be damaged by mud. At that price I’d want th shoes to last awhile. I’d wear inexpensive wedges or ballet flats I could walk in and not worry about ruining.
Party pooper! 
Nah, practical, after ruining far more shoes than I’d like to remember.
Im seeing Sept in “wine country” can include cool evenings.
lf, oh yes…can easily be 90 in the day, but 55 after sundown. I think that is precisely why they are having a 6pm wedding—so the nobody faints in the heat. Microclimates! A wardrobe challenge for sure! But a lovely family and I’m sure everything will be done to perfection…
Funny, @gosmom, but when I googled garden party formal, I ran across the same question on the wedding bee site. No one had any better idea than we do, here. They were just as annoyed, lol.