Why do some Eastern Orthodox churches use the Julian calendar for religious dates?

Some Eastern Orthodox churches continue to use the Julian calendar for determining fixed liturgical dates primarily due to historical and ecclesiastical resistance to the Gregorian calendar reform.12

When Pope Gregory XIII introduced the Gregorian calendar in 1582 to correct the drift of the vernal equinox, many non-Catholic regions—including Orthodox and Protestant countries—rejected the reform because they did not want to accept directives from the papacy. Consequently, while the Gregorian calendar became the international standard over time, many Eastern Orthodox churches maintained the older Julian system as a matter of tradition and independence from Roman Catholic authority.321

It is important to note that the practice is not uniform across the Orthodox world:
* Fixed Feasts: Some churches have adopted the Revised Julian calendar, which closely aligns with the Gregorian calendar, for fixed liturgical dates.1
* Movable Feasts: Nearly all Eastern Orthodox churches continue to use the traditional Julian calendar to calculate the date of Easter, ensuring that the "feast of feasts" remains consistent with the ancient methods established by the early church.14