you can't make both in 2021 as they will be happening, pretty much at the same time and anyone who can be, will be in guam

It's just the time that the 500th (or 400, have to check) anniversary of spain invading guam falls.
I know it seems weird to celebrate the date when your culture got subsumed by someone elses - but any excuse for a party right
Also it's celebrating the resurgence of the chamorro culture in recent years - driven in no small part by the sling and the traditions surrounding it.
Plus since spain sold guam to the United States for a glorified military base - the spanish seem to have kind of come back into favour somewhat.
Given all the slingers they have on the outlying islands, I think 2021 could well end up being the the largest gathering of competitive slingers in recorded history.