@ECmotherx2 — sure love that boiled wool coat. It’s very elegant!
If NJ is anything like PA, the public buildings will have already switched over to AC by April. The only time I could wear a sleeveless dress inside a building would be in July, when humidity levels are so high the AC can’t keep up.
But if the OP doesn’t trend towards being cold all the time, she will be fine.
Because a building with decent heating and AC has a range of temperatures, depending on the weather. In the summer, when it’s hot outside, indoor temperatures are likely to be in the range of 73 to 78 degrees. In winter, when it’s cold outside, indoor temperatures are likely to be in the range of 68 to 74 degrees. In other words, a building with good indoor temperature control will be colder in the winter and warmer in the summer.
Say that the temperature outside is a sullen 45, not at all unusual for New Jersey in April. Then the indoor temperature could be around 70. For most people, 70 degrees is too cold to wear a sleeveless dress and sit around.
Well, I must be the exception. 70 degrees inside - no wind involved seems fine for a sleeveless dress that is also a long dress.
With some of the suggestions above, I feel like we are trying to dress OP for sitting in her dress in a meat locker! 
(I know that some of the suggestions are for outside only jackets - but many references are for inside coverage - in which if you’re covering it up, maybe just pick a different dress!)
I tend to have more trouble with being too warm these days than too cool. Either an effect of getting used to colder climes or age 
Quiz time, people on this thread. Go look at your thermostat and see what temperature it reads. For you women, would you be comfortable in a thin sleeveless dress at the temperature you are now at? Would you be comfortable in a thin sleeveless dress if the temperature where you are was 70?
My thermometer says 69. I’m wearing long pants and a long-sleeved shirt. If I were wearing a sleeveless dress, I’d be reaching for the wool sweater. Then again, I run cold. YMMV.
My thermostat says 70. I’m in lightweight Prana pants that come to ankle with shoes and no socks. Shirt is a short sleeve T-shirt with a lightweight sweatshirt on. I’m perfect. I’d be a bit cold in sleeveless, but if I was outside and it was 70 and clear blue sky and sunny I’d take off my top layer.
Like some other women my age I can run warm. I would wear a sleeveless dress in 70 degrees in spring or summer but have a sweater or wrap. If dancing was involved I’d get hot quick.
I come from a different perspective as I was born in Ca, lived my entire life in Ca on the coast. Low to mid 70’s is my ideal temp. Once it hits high 70’s im sweltering. I think your body acclimated to what it is used to.
I’ll take back what i said about the dress being long - I looked again at the original post and see that I was mixed up with another dress. But I still stand - for myself - with being ok in a sleeveless dress in 70 degrees. I also wouldn’t object to a light something on top - not long - to have on hand.
This is a class reunion - isn’t there generally a lot of up and walking around, mingling and maybe even dancing?
My thermostat is at 67. I’m sitting right next to a fireplace with fire, wearing a below the knee heavy linen sleeveless sheath dress over a long sleeved silk tee and heavy synthetic slip, then a linen sweater, with a tissue weight wool shawl on top and heavy cotton knee socks, leather oxfords. I’m a bit chilly, and drinking a cup of hot tea.
I am never comfortably warm sleeveless inside. Winter or summer.
If I turn the thermostat higher, the forced air central heat gives me a headache.
On Saturday I was at a social event in a room that is not heated. They turned on space heaters for the event. I wore my warmest outfit. The outfit that usually makes me feel too warm in a normal environment. After an hour my feet were like ice and I was starting to get a sore throat. The other women wore polyester blouses with a sweater and seemed comfortable.
I’m wearing a sleeveless dress to a wedding in the middle of May in Cleveland. I’m wearing a jacket over the dress. Churches are always cold because they only heat them for events, and I doubt that they will heat this church in May. But it will feel really damp. The reception is outside in the evening. Depending on the weather, I’m changing into slacks and a sweater. At my age it’s all about comfort.
My philosophy is to layer … you can always take off a layer, and it’s better to have too many than chance being cold. I keep extra layers in my car just in case.
I went to an outdoor wedding and reception in Waimea on Oahu. I have some nice dresses but was concerned that it bay be cold so wore slacks and a nice top and brought a jacket as I really don’t like hives or goosebumps.
Turned out to be a good choice to have minimal skin as others were concerned about mosquitoes too! I was perfectly comfortable and felt I was dressed just right — honestly have difficulty recalling what I wore as a top.
Right now, I’m wearing capri leggings, sports bra, sleeveless tank, an old lime green patagonia fleece, and flip flops. It’s in the low 50s here today in New England - downright balmy! I do not like to be weighed down with lots of clothes - I want to be able to move.
Right now, I’m wearing wool socks, thermals, ponte slacks, thermal tank, warm wool icebreaker hoodie sweater and an unzipped insulated jacket indoors in Bay Area. It’s a cool 66 degrees.
Gourmetmom, your description made me so cold I just put on a wool duster and changed into uggs I use as house shoes for warmth. brrr
What is the temp inside your house?
Adding: 52 outside here, 67 in hall where thermostat is located, much warmer by fire.
Gourmetmom, you’re sitting outside wearing the capris, tank, fleece and flipflops in low 50s weather? Or you’re inside, and the low 50s is outside?
I just bought this on therealreal.
https://www.therealreal.com/products/women/clothing/coats/prada-short-collared-coat
I wear it when it is about 50-60 out. It feels very soft and very springy. It was like brand new.
Excellent buy, @oldfort !
I keep my thermostat at 68 when the heat is on and most of the time that is very comfortable. When it is in single digits or negative temps outside I will feel very cold, I think because my body is radiating heat to the walls and windows that are much colder than the air temp of 68 degrees.
Pretty coat oldfort!
I keep my thermostat at 68 too, but when it’s at 68 I don’t wear a sleeveless dress and bare legs.