My younger sister is getting married at the end of January, by which time wedding season will be in full swing, thus posing the eternal dilemma - what does one wear to a wedding?
As a guest, I always envy the bridal party: their outfits are sussed out so far in advance they've nothing to worry about. The cues as to what guests should wear should be on the invite. The venue and time will help dictate your attire. Church wedding? Reasonably formal; cover your shoulders; men should wear a suit. Garden, beach or vineyard? Slightly more casual, consider the ground when selecting shoes - wedges or chunky heels are best for sand and/or lawn.
If the wedding isn't until the afternoon and there's a formal reception afterward, you could be out late so be sure to take a jacket or pashmina to stay warm on the way home.
I've always felt like weddings are an occasion that warrants getting a bit dressed up for, so try to glam up beyond your norm. For me, that usually means I try to create a little drama by wearing a midi or full-length dress. The style and accessories are what defines your look as formal or more casual.