@Ataylor81 yes, just do not take a task. If you are solo, you will drop rank. You can start any day of the week and theoretically be placed with co-ops starting the same day. Mondays and Weekend Warriors are most competitive.
If you are in a co-op, you will not earn rewards if you do not race. Your co-op will be placed in a race NEXT week with the number of racers THIS week. This should go into your race/no race planning.