Christmas - red or white cropped cardigan partly tucked in, with flat shoes.
Work drinks - black bodysuit, black / nude heels and a sparkly bag. If you're going all out, a little fur jacket.
18th birthday - depends if out clubbing or in a family restaurant! Probably similar to work drinks, maybe some knee high boots underneath, a little dark grey silk top tucked in - or a blush pink silk top!
Also depends on your colouring!