For a formal or dressy occasion, a knee-length shift dress in a classic color like black, navy, or red could be a stylish choice. You could also opt for a floor-length gown in a softer fabric like chiffon or silk, or a pantsuit in a neutral color like beige or grey.
For a more casual birthday celebration, a midi or maxi length dress in a lightweight fabric like cotton or linen could be a comfortable and stylish choice. You could also try pairing a pair of high-waisted, wide-leg pants with a flowing blouse or a simple, tucked-in t-shirt.
If you prefer skirts, a pair of high-waisted jeans or a skirt in a darker wash could be dressed up with a button-up blouse or a sweater in a festive color like yellow or pink.
Regardless of the specific outfit you choose, it's important to consider the dress code of the event and dress appropriately. You should also take into account the location and time of the celebration, as well as your personal style and comfort level.