I read on this forum that Aegean Air had a sale mid-January on tickets to/from Athens to Santorini. Does anyone know if such a sale happens every month like it does with Southwest Airlines?
No, alas, it doesn't happen every month, with tickets from Athens to Greek Islands ... those just happen early in the year .... possibly some promotional prices will be added later for less popular islands, like Lesbos or Samos. There will be promotional prices in the Spring on Aegean flights from various European cities to Athens. But don't hang around hoping for Santorini price drops for any flights in warm months.
To track prices, on this link https://aegeanairlines.custhelp.com/app/answers/detail/a_id/302/related/1 click on the word "subscribing" to Newsletter (actually just an e-mail alert to any special pricing). The question "How do I find lowest fares available" directs you to "LOW FARES CALENDAR" .... so u could plan a trip according to lowest fare.
However, if you have a specific date you can just go to http://en.aegeanair.com/e-services/e-ticket/ And learn all the prices available that day. put in ATH and SANTORINI and the date, then click the box that says +/- 7 Days.... then it will show you 7 days either side of May 15... and what the lowest price is on each day. (BE aware that the bottom price is always the "Go Lite" price which doesn't include checked bag and is not refundable). On a date I just now put in at random, May 15, the lowest "regular" ticket is €67 (at 5:15am) and highest is €146 -- on the most popular flight, 11:55 am, the flight that is the best connection for N. American tourists arriving on morning nonstops.
In addition to Janet's comments we paid €38 each (Sept. 2017) for a 1300 flight from Santorini to Athens on Aegean. We checked our luggage, (free up to 23 Kg.) The caveat: We booked 8 months earlier for this bargain.
I traveled from Santorini to Athens one way in late September of 2017 and only paid $65. I bought the Ticket 4 months in advance. I used Skyscanner, to track airfares for a while. It mainly specializes in European airlines....You can just flag your date(s) of travel and it will notify you of airfare changes....