The stamp if from this series :
https://colnect.com/fr/stamps/stamp/638913-EUROPACEPT_Holidays-EUROPA_CEPT_2004_-_Holidays-Gr%C3%A8ce
of which I have one on this cover.
https://colnect.com/fr/stamps/stamp/192410-EUROPACEPT_Holidays_-_Ship_Voyage-EUROPA_CEPT_2004_-_Holidays-Gr%C3%A8ce
Thanks a lot George !