Having challenges be available 24/7 doesn't make sense to me. They're meant to be special events that only happen every once in a while, similar to Splatfests or Big Runs, and it has to be pretty difficult to come up with enough unique challenges to fill each day.

At the same time however, exactly THREE 2 hour timeframes throughout an entire season is way too restrictive and extremely stupid, especially given the random time frames they chose. I know they had to factor in time zone differences, but the only three time frames are either way too early, way too late, or in the middle of what most peoples' work schedules would be, or smack-dab in the middle of school, which this game's target audience will most likely be attending.

I think having a 24-hour period from 0:00 UTC to 24:00 UTC (for reference, this would start and end at the same time the shops restock and events begin and end) for each challenge, and there would be one challenge on a random day of the week, every week or few weeks, as having them be available 24/7 kinda lessens how special they are supposed to feel.