It’s quite easy to be modest and comfortable for that setting at the same time (and stylish, of course). I’ll describe something for a casual occassion - I’m not sure what you’re going to might be considered as - but if it needs to be more formal, it doesn’t take too much to dress things up, simply by omitting or switching out the obviously casual pieces.
A mid-length skirt (just past the knees) is the best bet if you’re going to be dancing, and it’s plenty modest. A full length skirt would be fine if you have one that’s designed for lots of leg movement. If you’re worried about being cold with a mid-length skirt, it’s rather popular these days to wear a pair of leggings or some other pants that follow the shape of your leg - I like to wear jeans under my skirts, especially in the winter.
That’s probably the easy part. Now you just have to find a top that will go with the bottom. Always remember the rules about patterns and solids and what not. I won’t go into that, I’m sure you can handle it
Don’t be afraid to wear a fitted t-shirt under a buttoned-down shirt. If the skirt’s casual, you could even wear a long-sleeved shirt - or a turtleneck - under a fitted t-shirt. (I say fitted because a t-shirt that isn’t tends to look a bit shabby, especially paired with a skirt.) I’m always the most comfortable when my arms, shoulders, and chest are all covered. It’s not hard to roll up the sleeves if it gets too warm, and it’s better to be covered up than to feel under-dressed, especially at a parish party.
When it comes to shoes, go with something comfortable. Flats, or even tennis shoes (not sneakers), unless your comfortable in heels. If you are, you are one lucky woman.
All that’s left are the accessories. Have fun