legal Definition of floor 1 a: a main level space (as in a stock exchange or legislative chamber) distinguished from a platform or gallery b: members of an assembly
Learner's definition of BIRTHDAY. [count]. 1. : the day when someone was born or the anniversary of that day. â often used before another noun.
When is your birthday is correct; everyone who has ever had a birthday party knows what a birthday is. But as Quora User points out, it's okay to ask what is your birth date (or birth month or birth year). The structure seems the same, but you are asking for a particular value for the date.