It damn sure better be layered and warm clothing as it can get damn cold in Maine in October and you could have snow, so don't wear shoes that are of such value or material that you would not want them to get wet, walking in snow.
I would ask bride and groom what they suggest. They might even tell you to wear jeans and snow jacket because of possible cold and snow.
Are you from a cold weather climate like New England has? If so you will know basically how to dress warm for that weather. If not, then you do need to talk to bride and groom or someone to tell you what you need (like gloves, thermal underwear recommended if outside and cold, and perhaps head cover. I lived up there (in Vermont) and have been over in Kennybunkport area in winter and it is cold as hell and it was snowing like crazy one time I was there in December.
Now some areas get snow more than others, so that depends on elevation and where in Maine you will be (Maine is a long state from North to South, with Bar Harbor being up in north end). So you might want to get more specific snow and cold projections for the specific area where the wedding will be.
Going to be a cold wedding outside.
P.S. TO GIVE YOU ANOTHER INDICATION OF HOW COLD IT GETS IN MAINE: I've spent time, multiple trips in motor home, on the Maine beaches in Southern Maine (Wells and Kennybunkport area which has a number of great RV Resorts and great lobster dinners). In mid summer, many people will go to the beaches, and I did too. The sand was cold and refreshing on the down side as you lay on beach with sun tanning your upside! Additionally, you will see very few people in the water, except those that wade out to below the knees and come back out ... very few will be seen actually swimming and I do mean VERY few. I actually liked it, as when you tan in Fla, the sand is hot and you get hot and have to go in the water to cool off ... in maine, just roll over and feel the cool sand.
If you decide to buy a jacket for up there, I suggest you buy it up there (or on line) to get a heavy duty one and recommend Columbia products. They are medium priced (say $200 ± depending on which one) for a heavy duty jacket that will REALLY keep you warm and have a lot of pockets and features that are nice. It will last forever, if taken care of. And you might get to use it in warmer climates with the zip out warm liner removed, but it will still be a big jacket. I spent plenty of times out in the woods and on the mountains, outdoor, in vermont and with my Columbia jacket and thermal underwear, heavy duty socks and good boots and good gloves and a cap that covered my ears, I sometimes would get warm and have to unzip my jacket and rarely got unduly cold (and I am essentially a "Florida Boy" from childhood.
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