Oct 1, 2021 through Oct 1, 2022 would seem to be certain and they have (recently at least) extended such anniversary celebrations for at least 3-4 months after the end of the actual anniversary year, so ending by Dec 31, 2022 (if not possibly including January 2023) would also seem reasonable. Historically, most of these celebrations have started on or (at most) 1-2 months before the anniversary date, but given that this is a major milestone year and they'll want to capitalize on the marketing as much as possible, pushing that time earlier to also include the summer season (for a total of two summer seasons to experience the 50th events) would make sense.