Cocktail attire preferred and bonus points if you wear celestial colors. Think: black, dark blues, purples, pinks, metallic. This is of course optional, but think about how good those photos will look ;)
There's no good answer as it's always variable. Generally though, it will be on the chillier side. Highs in the 60s & 70s during the day and lows in the 40s & 50s at night (sometimes cooler). CHECK THE WEATHER and bring back up clothes for rain and cold. Michigan is beautiful, but she can quickly change on you.
AS SOON AS POSSIBLE. Northern Michigan, especially Mackinaw, is the spot to be during the summer and fall months. People come from all over. Don't delay. Hotels and Airbnbs can book out months and months in advance. Act quick or else you'll be stuck paying more or stuck traveling farther.
Heck yes! Summer and Autumn are the best times to visit. Everyone loves summer, but Autumn is where the real magic is (in our humble opinions).