There are currently few cheap flights available with EasyJet and WizzAir with departures on Thursdays and Fridays.
The forecasts show pleasant 25°C on average in September in Santorini. It has also been reported that there are not so many tourists in Santorini at the moment due to the previous coronavirus pandemic, which still keeps most of the visitors at home. Due to this factor, it may make this picturesque island an amazing relaxing holiday.
As mentioned above, there are 2 airlines currently offering the cheaper fares to Santorini. You will find flights with Easyjet on Thursdays somewhat better and cheaper then the one with WizzAir as they include regular cabin baggage.
With WizzAir the flights are in the basic, cheapest class fare with all the extras such as larger cabin bag, checked baggage or seat selection optionable for an extra fee. For some may, however, be the Friday departure more convenient.
The choice is yours!