"They are going to organize the races on Sunday afternoon."
Translation:Ils vont organiser les courses le dimanche après-midi.
"le dimanche après-midi" means that from now on, every Sunday afternoon, there will be races. So, "on Sunday afternoon" is misleading.
An after thought: "they are going to organize the races on Sunday afternoon" can also mean that next Sunday will be the day when they will discuss the organization of the races (not necessarily when they will take place).
however the le should not be there unless it specifies that it will continue happening every Sunday afternoon.
just wondering if ranger can be used as well as organiser in this context. Thanks
No, because "ranger" is about sorting, tidying and/or cleaning.