I also want to know why it is so confusing even Roman, Greek, and Eastern Orthodox Catholics can't time Resurrectoin Day right. They celebrate two weeks after Protestant congregations because there is something abouto 14 days in the Gospels.
The difference is about calendars.
Catholics and Protestants use the Gregorian Calendar, a reform of the Julian Calendar that was introduced by Pope Gregory in the 16th century. While it was adopted pretty quickly by Catholics, it took longer for Protestant countries to fully adopt it. As to give an idea of just how recently in some cases, George Washington was born while the British were still using the Julian Calendar, as the British only adopted the Gregorian Calendar in the mid 18th century.
The Orthodox, however, still by and large still use the Julian Calendar.
The reason for the calendarial reform was because the Julian Calendar, while pretty good, ultimately meant that the calendar year and the solar year began to drift tiny little bit over the centuries. By the 16the century the drift had increased by about 10 days, and so two things happened to correct this. The actual calendar reform itself which accounted for the tiny portions of the day which weren't covered by the Julian Calendar. A Julian Year is on average 365.25 days long, 365 day normal years and 366 day leap years; whereas the Gregorian Year is on average 365.2425 days long, with 365 day normal years and 366 day leap years. Both systems have leap years every four years, the Gregorian reform is that this leap year does not occur on century years except ones divisible by four. This modification largely helps keep the calendar year in line with the solar year. The second correction was to literally remove days from the current year at the time of adoption, and so in 1582 October 4th was followed by October 15th.
The difference between the Julian Calendar and the Gregorian Calendar has continued to grow a little bit over the centuries, and today there's ~13 days of difference. As such it is common for days celebrated using the Julian Calendar to occur about two weeks after they are celebrated on the Gregorian Calendar. Easter is special because it is a moveable Feast, and so the timing between Easter/Pascha in the West and East can vary from year to year, sometimes it ends up happening on the same day, even though the dates are different on both calendars.
There's no actual disagreement between Western Christians and Eastern Christians about how to calculate the Paschal Feast, and so differences in timing really can just boil down to calendar quirks. The way we calculate the Feast has remained basically the same since the Council of Nicea which produced a standardized method of calculation.
-CryptoLutheran