First you need to know the level of casualness of a dinner party. Casual is a little below semi formal and covers pieces like: flowy dresses, slacks, polo shirts or jeans. These are few classics you should have and you are set to go. Casual does not mean the simple clothes you wear at home or while going out for groceries. See what the venue is and then decide how much you want to dress up.
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Men have the liberty to wear all kinds of pants except sweatpants. You could go with Dockers, dark jeans or chinos; all are suitable for the event. The main idea is what you pair the pants with. A dress shirt will compliment jeans very well but will not look good with Dockers. You could also wear a casual t-shirt or a polo shirt with slacks; do not wear them with jeans as it will be too casual.
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Woman should follow the same rules. They could choose skirts or flowy dresses and pair them up with wedges. You could also go for jeans with a stylish top and heels. Do not go for these accessories if you are wearing a skirt. The accessories should be chosen according to the dress. If the dress is towards the formal side; use minimal accessories, if it is more casual than accessorize it. If you are wearing jeans, pair them with more formal shoes. Sandals or ballet flats are great for such occasions. You can also pick up a clutch to give chic style to the jeans or if it is a skirt, go for a simple, larger bag.
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The hair and makeup also determine the casualness of the evening. Try to keep them simple. It is OK to carry an up do if it has some locks flowing and a messier look. An everyday ponytail will also work fine. If you are making heavier eyes, use a very neutral lip color and vice verse.
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