The Orthodox Church Calendar
The Orthodox Church's calendar is a fusion of solar and lunar calendars or calendar cycles.
For most of its early history the Church did not have a single solar calendar. Rather, in the year 325AD, because each locality was on the solar calendar of the land (either the imperial, the egyptian, the persian or the armenian calendar, depending on which locality you were in), according to various saints, including St. Athanasius and St. Ambrose, the council of Nicea created the 19 year formula that fused the Hebrew calendar with any local solar calendar resulting in the same date of Pascha, and thus the same Pentecostarion and same Sunday observance (Octoichos) throughout the year. This can be seen in that both today's Old Julian and Revised Julian observances both still result in the same observation of Lent, Pascha, Pentecost, and same weekly and Sunday cycle of tones and hymnography (Octoechos/Paraklitiki). Thus, back in the fourth century, while in some localities Christmas was celebrated on December 25, and in others January 6, and different monthly observances, the weekly cycles including the Sunday cycles, Lenten, Paschal, Pentecost, etc. were all the same.
This, again, is because the Church calendar is a fusion between two kinds of calendars, namely the solar or monthly cycle (which for much of history has had varience of observance), and the Ecclesial-Hebrew lunar cycle (which for most of history has been universally observed, but especially since Nicea). The Church's Ecclesial-Hebrew lunar cycle is not dependent on the observances of the non-Christian sects of Judaism that have changed their way of observing feasts since the early centuries AD.
The Church today observes its lunar year (thus its weekly cycle and Paschal cycle) all the same, except Finland which is forced by the government to observe it based on Gregorian calculations. The solar year is observed by some local churches based on the older Julian reckoning of leap years, and by other local churches based on the revised Julian reckoning of leap years. While the revised Julian reckoning is often called the "new calendar," it is only a solar part of the calendar and is distinct from the Gregorian reckoning in that it uses an entirely different formula (and is more scientifically accurate in its formulation of the solar calendar than is the Gregorian formulation). It also places each date of the year closer to the places in the year that they were observed in the early centuries of the Church (the old Julian reckoning of leap years had each date recede by 13 days from that time until now).
Why is Orthodox Easter sometimes the same and sometimes different than Catholic/Protestant Easter?
For the Orthodox Church, every Sunday is a weekly Easter/Pascha. It is the weekly celebration of the Resurrection, just as every Friday is the weekly commemoration of the Lord' s crucifixion, every Saturday the Lord's descent to death to conquer hell and the grave, etc. Yet, we also have the yearly commemoration of these things, based on a formula that blends the Hebrew lunar calendar with the Julian solar calendar, which recede from the astronomical solar year at about the same rate.
The Orthodox Church follows the older calculation that was universal before the “Gregorian calendar reforms” of the 16th century, based on the “19 year cycle” established by the First Council of Nicea, and tables of dates that were established a few centuries after that. The tables set dates for the annual celebration of the Resurrection, or Pascha, such that the Sunday of Pascha fell on or immediately after the 17th of the Hebrew month of Nisan (i.e. on the Sunday after 15 Nisan, Hebrew Passover as it was determined in the 1st century). If the Passover happens to fall on that weekend, another week passes. Christ rose from the dead on the Sunday following the Passover, and He is the eternal Passover, thus the importance of the connection.
*Note, the audio readings above are provided by Ancient Faith Radio and provides readings based on the Revised Orthodox Julian Calendar, because a majority of Orthodox jurisdictions are on the Revised Julian Calendar, as are the large majority of Orthodox in America. While most of the year the daily readings are the same for the Julian and Revised Julian Calendar, there is a portion of the calendar where they differ. For this reason, we here provide the readings for days when they differ.
The above audio readings are provided by AFR and are based on the Orthodox Revised Julian Calendar because a majority of Orthodox jurisdictions (and the largest three in the US) follow the RJC. There are no audio readings based on the old Julian reckoning of the Menaion.
The Orthodox Calendars (both Julian and Revised Julian) are both the same with regard to the Octoechos, the Lenten Triodion, the Pentecostorian (Flowery Triodion). The only difference is that observation of the Menaion (currently 13 days apart) and "shifts" that occur with regard to the Menaion (such as Lucan jump, and when major feasts fall on a particular day).