Check date availability on Google Flights and use those dates to book on Priceline , free cancellations within 24 hours of booking
Please note that while this fare is valid at time of posting, fares can fluctuate in a matter of minutes/hours/days.
My sample travel destination/dates:
BOS-ARN-BOS Feb 11-18 (1 week) - makes one stop in KEF (avg1 hour layover) $379
BOS-CPH-BOS Jan 5-21 - makes one stop in KEF (avg 1 hour layover) $381