You only need to avoid the anchorage a regatta is using on a specific night. They actually suck boats away from the other anchorages leaving things wide open.
Agree George. That is why keeping note of when they are occurring and ideally knowing the itinerary would be good. I’ve encountered several large flotillas at times and like regattas while they are great for those involved, they suck mooring availability and restaurant resources so I just play avoidance.