The deadline for admission is February 1. If you want to apply for a graduate assistantship you must also do so before February 1.
The department does not admit students at any other time except very rarely under special circumstances. Ignore any other information that states admission deadlines other than that above.
It takes a minimum of 30-60 days for the university and department to process applications. You should receive notification on the status of your application at the end of that time.