How is this possible?
 

I was scheduled to fly from YYZ to DCA but because of the snow at DCA, I rebooked the flight for tomorrow (2/18). When I checked my booking online, I noticed that my flight had been rerescheduled for 2/19. I called AC and asked them why they changed my booking again. They said that DCA had informed them that the airport would be closed on 2/18. But according to the news reports, the airport still "plans" to open at 7am on 2/18. Is the airport just banning AC flights tomorrow? What's going on? Thanks.

it is possible that they are limiting service into the airport and gradually allowing airlines to fly back in as opposed to having them all fly in at once.

Other possible reason is that the plane they planned to fly you to DCA on is still stuck somewhere else and won't make it back to YYZ.

I have found lately that AC seems to cancel flights quickly, for questionable reasons. I have begun to believe it is a method for reducing to capacity, increasing seat miles. IE, they have 75 people booked to DCA on the Airbus, and 45 on the next flight, if weather will let them get away with cancelling one and operating full, I believe they try and do it. Others may say that really isn't possible with aircraft needing to be in certain places, crew sched ect... but I wouldn't be suprised.

The airports may be open but they are restricting landing/take off times heavily. Their runways are in terrible shape not to mention the aprons. Also many of the staff are unable to leave their homes to even get to work. And the weather still isn't all that great.
As far as cancelling flights to try and operate one full one, whoever it was (can't remember) that said planes and crews all had to be somewhere is correct. We don't just cancel on a whim. Many times downline stations are experiencing air traffic concerns and cancel an airline's landing slot. When that happens we have no choice but to cancel a flight, get everyone out on another and use the slot that has been left for us to accomodate that flight. Flight attendents, pilots and even the airplane all have schedules and are expected somewhere else so to cancel a flight often means a plane has to be ferried to continue it's schedule or another one rerouted.
Happy and safe travels!