Were all commercial flights to and from Washington, D.C., canceled on October 11, 2021? No, that's not true: There is no evidence that happened. Flights still were moving as planned in the morning and early afternoon on October 11, according to the Reagan International Airport arrival and departure page, as well as the Dulles International Airport arrival and departure page on their respective websites.
All commercial flights cancelled in / out of Washington DC Monday 11th
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This is what the blog post looked like at the time of writing:
(Source: Simonparkes.org screenshot taken on Mon Oct 11 3:28:26 2021 UTC)
(Source: Flyreagan.com screenshot taken on Mon Oct 11 3:47:11 2021 UTC)
(Source: Flydulles.com screenshot taken on Mon Oct 11 3:47:11 2021 UTC)
Here is also a link to the Automatic Dependent Surveillance-Broadcast (ADS-B) Exchange website. The ADS-B website is "the world's largest public source of unfiltered flight data" according to its home page. On October 11, 2021, flights in and out of Washington, D.C., could be seen on ADS-B's system.