How will planning to do a mario kart event beforehand work better than easily making it the same day?
What is there to plan anyway? It's not really a tournament, and is more of a community that has ranking. You can still play after you completed the number of races.
Situation 1:
"Hey guys everyone play MK7 right now we're gonna do some races"
2 people show up because everyone else with MK7 may not be on the forum at that time. When they get on the forum a few hours later, they see the community. They attempt to join it, and it's a ghost town.
Situation 2:
"Hey guys, on August 17 at exactly 5:42 PM CST everyone join this Mario Kart community, we're gonna do some races"
16 people show up because everyone has had time to read it.
I will join the MK7 community in the next 30 minutes and I will not get a group. If I do, it's going to be me and one other person - most likely you.