Snape arrived to the party fashionably late - never let it be said he hadn't learned the social niceties from Lucius and Regulus.
In Draco's honour, he'd gone to some effort with his costume. He'd even brewed a hair-growing potion for the occasion, and was now sporting a painstakingly-trimmed goatee. For some reason, the costume had originally been in entirely-unacceptable Gryffindor colours, and Snape had wasted no time transfiguring it to much more suitable Slytherin hues. He thought of the cobra-headed staff as an affectionate homage to Lucius' cane.