We don’t normally get told in advance what date the anniversary event is going to kick off. It was 20th June in 2018, 18th June in 2019, which were the penultimate weeks of June, but given that this year June 30th is a Tuesday, it’s possible that it could be around the 23rd, or maybe a week earlier around the 16th.
The player organised MEGAversary is starting on the 24th, but there’s no guarantee that the official event will share the same dates.