Dress to please yourself, and do not worry: think of this more along the lines of a comfortable chat with a series of old friends one has not seen in quite some time, rather than a rite with defined magical goals and outcome. The funereal rites for the newly-dead are more precise, and have additional requirements, but these bimonthly observations are more in the way of reminding the honoured ancestors that they yet live while their names are still spoken. (And, I must confess, my own version of the rites is entirely influenced by the Christian observations my father taught my brother, and my brother passed on to me; both ends of the matter would consider me entirely heterodox, but I find the blending to be pleasing, and the honoured ancestors have never made their displeasure known.)

Shall we say half five or so? And should you feel more comfortable bringing one of your friends with you -- it can be awkward to encounter one's professors outside the familiar Hogwarts environment -- you are of course welcome to do so.

In service,
Antonin