I am planning a trip to Greece next April and all indications are that numbers of tourists and prices go up after Easter. Are we talking Christian Easter (4/21/19) or Orthodox Easter (4/28/19)?
After "Western" Easter. It is tourists from Northern Europe who head south. Tourist numbers go up OVER Easter. as people go on holiday incorporating the 4-day Easter break and associated school holidays. Then they drop a bit, but stay higher than before, and increase significantly when the summer school holidays start.
Same for the whole of the Mediterranean area.
P.S., both They are both "Christian", Western (=Catholic+Protestant) and Orthodox.
Last year when I was planning my trip, the prices jumped about 25-30% on May 1. Both Orthodox and Catholic Easter were on April 16.
BTW the Orthodox are Christians -:)