So, what is Easter and why is it a big deal?
Easter is a religious holiday that is the culmination of Holy Week in Christian religions. In Christian religions, Holy Week is the week in which Jesus Christ held the Last Supper, was crucified on the Cross, died and was entombed, and then, three days later, arose from the dead.
This sequence is known to Christians as the Crucifixion and the Resurrection. It is the beginning of the Easter Season, which begins on Easter Sunday and continues for the subsequent seven weeks. Holy Week is preceded by Lent, a period of six weeks that commences on Ash Wednesday.
Easter occurs on different days every year, and thus is known as a 'moveable feast.' The date does not derive from history, but rather from church doctrine. It is not a creation of Rome, but rather of the Eastern Orthodox church. This was a time when the two churches were joined, before the Great Schism of 1054.
In the year 325, the First Council of Nicaea determined that Easter should fall on the first Sunday that occurs after the full moon that occurs on or soonest after 21 March - which often but not always is the spring equinox. They thought 21 March was the date of the Spring equinox then, and it is as good as any other choice anyway. It ensures that Easter occurs during the early spring, which was the point aside from religious purposes.
The earliest date for Easter is 22 March, and the latest is 25 April. This is because the lunar cycle (28 days) is determinative and will not last past the latter date without there being a full moon and a subsequent Sunday. That is the same now as it was then, and they fully understood the phases and cycles of the moon during Roman times.
In practice and secular terms, Easter is a general mark of the end of winter and the first bloom of spring. It is celebrated with Easter Eggs which are delivered by the Easter Bunny. Naturally, as a solemn religious date, there are church services and Easter parades.
Easter is the holiest of the Christian holidays, and observant Christians rank Easter as a more important religious holiday than Christmas or any other day on the calendar. That is somewhat debatable, but most Christians would agree with that assessment, though there is no question that Christmas has become the bigger secular holiday. The calendar dates of both Christmas and Easter, incidentally, are both rooted in ancient customs and decisions and not by the dates of actual historical events. They are symbolic dates in lieu of knowing precise dates for the actual events being celebrated.
Easter is not always celebrated on the same day everywhere. The Roman Catholic and related churches use a different calendar than the Eastern Orthodox Church, and the two do not coincide in terms of determining the date of Easter each year. The Julian calendar was promulgated by Roman Emperor Julius Caesar well before the birth of Christ and marked a significant reform and improvement of the archaic Roman calendar. Some Eastern Churches still use the Julian calendar, which is quite good for a calendar that is over 2000 years old. However, most churches, including all those in the Roman Catholic sphere, switched to the more modern (in relative terms) Gregorian calendar in 1582 or at some point thereafter.
The Gregorian calendar was the result of hundreds of years of observation of the Julian Calendar and how it worked well in some ways and not so well in other ways. The Gregorian calendar matches the actual solar/earth cycle slightly better than the older Julian calendar, though the differences are slight - the Romans under Caesar did a very good job figuring out their calendar, and could still be used today, though the seasons would fall in slightly different places. Those churches using the Julian calendar follow the same reference points for Easter, i.e., it falls on the first full moon after 21 March, but their 21 March falls on 5 April in the Gregorian calendar. The Julian calendar is said to be 13 days 'behind' the Gregorian calendar.
Thus, the Julian calendar Easter may take place weeks after the rest of the world celebrates it - if the full moon takes place after the 21 March day of the Gregorian calendar and before the 5 April date. In that case, the Julian calendar churches would celebrate Easter a month after the Gregorian calendar churches, after the next full moon. The churches also, though, can celebrate Easter on the same Sunday, if the full moon falls after 5 April as marked on the Gregorian calendar.