I am a 4th year Math and CS undergrad with definite plans to enter graduate school. However, I know that I want to take a year off between graduation and grad school, but I don't know if I can apply this year and take a leave of absence or something like that. Should I not apply and apply after my year off?
keshlam truly mentioned, you need to check the existence of any deferral plan for that program.
If your admitted position would be a course-based one (like MEng), the deferral possibility would be applicable, noticeably. But in the case of the research-based programs (like M.Sc., M.A.Sc., Ph.D. and so on), the deferral chance might not be high, because such positions are often defined based on the funding, provided by either industrial or governmental customers and would not be postponed, considerably. If so, you have to reconsider to apply.