Skyscanner has this. You don't even need to specify a city: These are the results for 'cheapest month' and 'everywhere' from DC https://www.skyscanner.com/tr

Skyscanner has this. You don't even need to specify a city: These are the results for 'cheapest month' and 'everywhere' from DC https://www.skyscanner.com/transport/flights-from/wasa/?adultsv2=1&childrenv2=&cabinclass=economy&rtn=1&preferdirects=false&outboundaltsenabled=false&inboundaltsenabled=false https://www.skyscanner.com/transport/flights-from/wasa/?adul...