What sort of thing?

It's for the boys to give the girls lingerie ... (preferably white!)

when is White Day? 

Can't remember... ^^;

Can you give valentine's chocolate or a white thingy to a friend, or just
someone you love/lust/infatuate after?

No idea, but it's generally the girls giving it to their boyfriends or
boys they have a crush on ...

Kun-chan...