Most programs are now accepting applications for 2020 only. Each program summary lists deadlines on its Apply tab.
Fall 2019 applications
If you are still interested in admission for Fall 2019, which begins August 26:
Degree or certificate programs: Most programs' Fall 2019 deadlines have passed, so consult your department before proceeding.
A deadline listed as "rolling" means the last day for applications and admissions is August 23, the Friday before classes start. It is rarely feasible to apply and be admitted on the same day.
Post-baccalaureate non-degree: Applications will be accepted until August 30 (the end of the first week of classes).
Deadline extensions may be granted if the courses you intend to enroll in are mini-courses that start later.
If your application for Fall 2019 is not completed by you and reviewed by your department soon enough for Fall 2019 admission, you may end up wanting to be considered for 2020 instead. In that case you will need to reapply for 2020.