Most Christians will be following the Easter dates in the Gregorian calendar, which was introduced in 1582. But the Eastern Orthodox Church still bases its Easter dates on the previous Julian calendar, which had been in effect since 45 BC.
As a result, Greek Easter or Orthodox Easter can vary from the main Easter celebrations in the western world. In some years, both are on the same date, as in 2017. In other years they aren't. In 2021 they were four weeks apart, although the two festivities can be separated by as much as five weeks, so it's not the biggest gap that could potentially happen.
The dates for Greek Orthodox Easter in 2023 are as follows:
Good Friday - April 14, 2023
Holy Saturday - April 15, 2023
Easter Sunday or Easter Day - April 16, 2023
Easter Monday - April 17, 2023
This year, that's a week later than Western Christian Easter. Despite being commonly called Greek Easter, this tradition is celebrated in other places apart from Greece. Other countries that officially observe Orthodox Easter include Romania, Bulgaria, Ukraine, Serbia, Moldova, North Macedonia, Cyprus, Montenegro, Georgia, Belarus and Russia.