Flights from Washington DC to Athens

I would choose Turkish Air. It flies non-stop overnight from IAD/ Dulles Airport (and maybe BWI too, I'm not sure). You'll connect to Athens via IST (Istanbul). Turkish Air is a great airline all around, very good prices and service. You'll have a connection no matter what, so I would definitely pick Turkish Air for this itinerary.

Agree with Agnes about Turkish. Great airline. Surprisingly comfortable seats, generally very good service.

Turkish does fly from IAD nonstop to Istanbul (they do not fly to BWI). Personally, I always try to maximize the transatlantic leg to allow a better chance of getting some decent sleep - going via IST will help with that. Frequent connections from IST to ATH.

If it were me, I'd try to get a flight on Turkish.

Why do you avoid British airports? We always fly into LHR and transfer, easy with Heathrow Connect link.

Since I'm from Philadelphia no surprise that I most often use American nonstop from PHL. I used to joke (grimly) about (back in USAir days) the torture-trial aspect of economy seating, but this last year they must have reupholstered, because v. comfortable, and in an aisle seat I even snoozed about 3 hrs. The advantage is PHL to ATH is really just 9.5-10 hrs flying time ( 4:30pm - 2:15 am + 7 hrs clock-change), and it's always always on time, arr 9:15 in AM. That's even if its delayed in Philly due to runway traffic... it has all that Atlantic-Ocean time to make up for it.

It all depends on how much hassle it is to get to Dulles vs. PHL. Frankly I like the idea of, once I leave US, there's no connection overseas to fret about. I've done Heathrow (oh my, they don't announce a gate until 30 minutes before departure, and you run insanely down corridors, jump on a train etc etc), Chas D Gaulle (sheer hell; never again), Frankfurt & Zurich (both OK) and JFK Delta (a hassle in NY + higher prices). Haven't done Turkish air & I hear the economy seats are decent ... it would take a bargain price for me to consider (mainly because of Turkey's benighted government, oppression of women, bad human rights etc.) but that's just me.

I personally think it's too early to really engage with buying tickets for Sept-Oct 2020, but starting to track prices maybe 8 months out is a good idea. I've paid between ~ $600 - $780 round-trip on Turkish Air on 3 flights that I took from Dulles Airport (Economy Class, which was fine). Turkish Air often has seasonal sales (so check around). You should have no problem getting a decent fare at the time you're going since it's not exactly peak travel season.

If you want a nonstop flight, your only option from the airports you listed is American from Philadelphia to Athens. I'm not sure when it stops running for the year, though (it's "seasonal" and doesn't run year-round).

From Baltimore BWI, you don't want to go via London, which is their only nonstop to Europe. But you can go via Canada, if you don't mind Air Canada Rouge (which gets very mixed reviews). You would connect in Toronto or Montreal. Or you can go on American via Chicago. Again, these flights are seasonal.

From Washington IAD, you have the same choices as from BWI. In addition, you can go with one change via a wide variety of European airports. Others have talked of Turkish Air (I like them too); if that doesn't suit, look into (for starters) Lufthansa via Munich or Frankfurt, Aer Lingus via Dublin, KLM via Amsterdam, SWISS via Zurich, SAS via Copenhagen...Many of these flights are year-round, so this is a good option if the other flights don't work with your return date.

A great trick I learned on this Forum: to find who flies where from an airport, look at that airport's Wikipedia page

My daughter and I are taking the RS Athens and Heart of Greece tour in April 2020. We are doing round trip out of Washington Dulles IAD on Lufthansa to Athens, and will be changing planes in Munich. We could have changed planes in Frankfurt but I preferred Munich as it’s a smaller airport.

We flew Emirates non stop from Newark to Athens this past October.

Dulles, Lufthansa via Frankfurt and Munich are both a breeze.

I would personally avoid Turkish airways at all costs, Philadelphia to Athens direct flight is one of the best choices if you want to avoid the stated airports. I'll be honest we flew Iberia airlines into Madrid and it was a breeze, well organized and super fast. On the way back we flew Santorini to London's Heathrow with a few days to stay there. I love British Airways, they are so much better than Turkish Airways, in service and cleanliness...

We have taken Air Canada rouge several times. Its great to arrive Athens mid morning with that direct flight. Rouge certainly has its critics however we fly premium economy. It is essentially a business class service in a separate cabin area. Food and drinks and service is good. The only negative is the seats while are wide they do not recline so you have to sleep siting up. I did wander back into the main area of the aircraft. The leg room was much bigger than I anticipated but I suspect the food and service is probably not the greatest back there. On one of our trips on Rouge we got into a discussion with the Premium class passengers. It turned out that half of them were Americans taking advantage of the substantial exchange rate between US and Canadian dollars.

We too prefer Munich over Frankfurt. One thing to remember though you are entering Europe when you land in Munich and while you do not have to collect baggage ( hopefully you are booked on a through flight and will get your luggage in Athens) you do have to go through passport control. After that process you end up outside the secure area and have to go back through security which can take some time.
We have never had a problem when we had a 2 hour window for this whole process even in Frankfurt.