You can buy a roundtrip flight from Washington, D.C., to Stockholm, Sweden, for $456 right now, according to The Flight Deal. But as usual, availability is extremely limited, so it's in your interest to book fast. (TFD says it's likely to be gone within a couple of days.)
The United Airlines fare is valid for travel from late October to early December, and you're required to stay for a minimum of seven nights. The flight includes a stopover in Amsterdam. You must buy your ticket at least 28 days before departure.
To check for availability, TFD recommends using ITA Software Matrix Airfare Search and inputting the following criteria:
Pro tip: Wow Air is also offering competitive roundtrip prices for flights to Stockholm, which start in the low $200s if you're flying from Washington, D.C., this fall. All Wow Air flights to Sweden include a layover in Reykjavik, Iceland. If you'd prefer to fly nonstop (and almost as cheaply), we found roundtrips starting at $279 on Norwegian Air from New York City on Google Flights.