Since the course is online, you can begin ANY time. Typically a Cycle of study begins at a particular season, however, even if you miss that season, you can merge at a later stage.
If you join toward the end of an existing cycle, you will subsequently merge with the new cycle when it starts.
For example, Cycle 14 begins mid 2023. If you happen to join in 2024 or 2025, you will be able to enjoy everything that Cycle 14 offers, but then also continue into Cycle 15.