Winter weddings ask for two things at once: something that looks like an occasion indoors, and something that survives twenty minutes outside for photographs. Most people solve the first and forget the second.
Start with the coat
It is the piece people see first and last. A longline coat over a dress reads more considered than a short jacket, and it covers the whole outfit while you are waiting outside.
Sleeves are worth more than they look
A long-sleeve midi in a dark tone is easier to wear all evening than a strapless dress plus a wrap you have to keep adjusting. If you want something sleeveless, plan the layer that goes over it before you buy.
Length
Midi is the safest choice for a winter wedding: long enough to feel dressed, short enough to move in. A maxi works for an evening reception; a mini needs opaque tights and a plan.
Shoes over style
Gravel, wet grass and stone steps are all likely. A block heel or a heeled boot handles every one of them; a stiletto handles none.
For men
A blazer in something with texture reads warmer than a flat suit jacket and photographs better. A roll neck under a blazer is a legitimate alternative to a shirt and tie at anything short of black tie.