I was referring to you saying it takes 20 episodes to see whether a series is good or not and/or the plot gets kicked into overdrive. I heard elsewhere it's 13.
Oh no, I mean for some reason in each season, episode 21 is the specific episode in which something seriously awesome or plot related starts happening.
Adventure 21: The Mamoru Hosada directed episode in which Taichi goes back to the real world and he realizes how much crappier things have gotten.
Adventure 02 21: Ken is defeated by Magnamon ending the Kaiser arc.
Tamers 22: (I could say 21 but that was the Leomon debut episode and 22 would work better here) When the Pig Deva arrived and when the entire city was being destroyed. Also, they used the Juggernaut program again, which
really screwed up the Digital World/Real World barrier.
Frontier 21: Duskmon first fight. Owns everybody.
Savers 21: Kurata debut. Also was the episode where SaberLeomon starts his final attack on the real world.