No. Technically, it never has. National Airport opened in 1941, and has operated ever since as a 24/7 facility. It did, however, adopt the Nighttime Noise Rule in the late 1970's (IIRC), which forbade operation of certain aircraft between 10pm and 659am. I moved into an apartment in Crystal City - less than a mile from the north end of the runway - and recall the parade of takeoffs beginning at 7am every morning.

Everyone referred to it as a curfew, because the jets at that time exceeded the allowable decibel levels, and, thus, flights stopped landing at 10pm and takeoffs didn't happen until 7am. But......

The airlines - and passengers - hated the curfew which often meant flights diverted to Dulles. And Boeing and Douglas developed hush kits that allowed jets to land and take off more quietly.So the FAA made it clear that jets that met noise standards could operate 24.7 at DCA.

Now, with the changes in aviation technology, certain full-size jets regularly land at DCA late into the night. And they take off as early as 515am, rattling windows and making life miserable for Georgetown residents who are directly under one of the flight pads but are prevented by Historic Preservation standards from making affordable soundproofing improvements to their homes.

So, no. There's never been a curfew. But there have been restrictions. And the airlines have found ways around the restrictions.