Update 2021-04-19 (2): As announced a few days ago, the quarantine requirement for travellers from the EU, the United States, the United Kingdom, Serbia, the United Arab Emirates and Israel was lifted on Monday (19 April). From now on, only a negative PCR test (not older than 72 hours) or a COVID-19 vaccination that has already taken place must be proven.
Update 2021-04-19 (1): According to media reports, Greece has changed its regional risk classifications. The islands of Thassos and Kythnos, the municipalities of Rethymno, Phaistos, Maronia, Didymoteicho, Pydna, Kavala, Volos, Sintiki and North Tzoumerka are classified as very high-risk (dark red). Likewise, Mykonos, Leros, Karystos, Kymi, Skyros, Lokres, Agistri, Kythira and Hydra have been classified as high risk (red). For details on the restrictions in “red” and “dark red” areas see here (in Greek).