For me, thinking about a festive/dressy capsule has been way more productive than trying to plan outfits for specific events.
I started by listing events ranging from dinner at a nice restaurant to performances at jazz clubs and theaters to Christmas parties (a.m. and p.m.) to having friends over during the holidays. In my case, I figured I could cover most of these events in with a pair of silky, skinny trousers, a couple of skirts (one pencil and one flowy), a soft cardigan, three tops (edgy, sparkly, lacy--with at least one three-quarter sleeved), and a silky, shirt with a bit of detail that didn't look like a business shirt. I also figured that using a very basic color scheme of ink/cobalt blue and black for these items would let me pair these pieces with my other clothes to create even more variations. By switching in accessories, boots, and dressy shoes, I can cover most of the dressy events in my life without feeling like I am always wearing the same outfit.
I found several of my capsule items existed in my current wardrobe (pants, silky shirt, soft sweater, and pencil skirt--all in black) and purchased the other pieces in a single shopping trip. Since I was looking for specific types of items, I found I could cruise through the stores quite quickly. I also found shopping for tops and a flowy skirt quite a bit easier than trying to fit dresses and tailored pants. I substituted items if I found something I liked that could work in the same way as my original thought. My flowy skirt turned into a midi with an asymmetrical hemline, and my "lacy" top into an edgy, ripped, cowl-necked, three-quarter sleeved top in a ink blue/black print. Since most of my dressy items don't get worn that often, I wasn't as concerned with quality since it's unlikely that most of my dressy items will be worn that frequently; if they last a season or two, that's just fine with me.
I think Angie is quite right in saying that it makes sense to have a capsule for specific activities that crop up regularly in our lives. Trying to create an outfit every time an event is upcoming is hard work and, to me, often discouraging. My capsule lets me play around a bit with the items to create outfits that harmonize with how I want to dress for a specific event. I can do classic or edgy, casual or dressy, or any variation in between with my capsule. My capsule has taken the stress out of dressing for these events.