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When flights are cancelled, it's quite rare for all the same passengers to be rescheduled en masse on a brand new (previously nonexistent) flight the next day. Airlines don't have the spare aircraft and crews to do that sort of thing. Passengers are rebooked on whatever flights are available, and some may have to wait several days. Some passengers will just decide not to travel at all.
If there are ATC delays due to volume, it's usually of one of a few reasons:
In other words, when there are volume-related delays, it's not always the case that there are significantly more flights coming to that airport, but that the airport is unable to handle its usual volume of traffic.
Short answer: more passengers per flight.