They seem to have just ran a batch of them that's bigger than their usual schedule change Saturday. (And Delta tends to mess with flight schedules more than other airlines to begin with)
The good news is that Delta is generous with rebooking you as long as you're within reasonable parameters- seats available in cabin class you paid for and less than 24 hours +/- original flight times. The general rule of thumb is that Delta allows one free ticket change if:
*a flight departure time changes more than 60 minutes
*a flight arrival time changes more than 60 minutes
*your new itinerary would put you below Delta's Minimum Connection Time for a given airport and connection type (domestic to international, international to domestic, domestic to domestic have different MCTs as do international pre-clearance airports)
One nice thing about the policy is that if you booked a route that you weren't crazy about (pre-dawn departure, late arrival, connecting in an airport you hate, etc.) because it was significantly cheaper than the 'good' flight times, you can sometimes use the schedule change as a way of getting flights you like better at no additional cost to you with your free change- just research and know the new flights you want when you call in with your change request.