It is recommended to start preparing your application as soon as possible. After all, the faster you get into a university, the quicker you can plan your trips. To avoid disappointment, make a calendar note of all the essential application deadlines (as specified by your chosen university).
Application deadlines will vary by university, but for programs beginning in the fall (September/October), applications will typically be open from early in the year (January/February) to the middle of the year (June/July).