It comes from when europe changed to the gregorian calendar (the calendar we use now) In order to make the change, we had to 'loose' about 10 days. So our calendar jumped from 1st April to the 10th (don't actually know the dates, but you get the idea)
The orthodox church, and also most of the 'east' didn't changed they continued on the old calendar. The countries eventually changed but the church didn't. So orthodox churches celebrate 'new year' and christmas and easter etc all according to the old calendar.
That is a somewhat simplistic version, but just the general idea.
Ummm... It has nothing to do with calendar changes or indeed Christianity in this country. 'Easter Sunday' here is defined as the first Sunday after the the full moon following the Spring Equinox. Regardless of what date it happens to say on your calendar, Easter is calculated based on the moon and the sun.
Actually the difference does relate to the diffence in the calendar, because the spring equinox is defined as 21 March, not as the actual day and night which are in fact equal.
Also the 'ecclesiastical full moon' which is used is also not necessarily the actual date of the observed full moon but is worked out according to a table which is based on lunar months, not solar ones. The Gregorian and Julian calendars use different ways of correcting the difference between the lunar and solar calendar, so sometimes Easter is calculated using a different full moon, because it has to be the one that falls next after the equinox.
Disclaimer: this is a very basic simplification of a complex method of calculation.
Not it's not, the spring equinox can occur between the 19th and 21st March. The calendar is irrelevant, you could create a new calendar and call it the 15th of Flibflab and the equinox would still occur at the same time, when the point of the directly overhead sun falls on the equator.
If Easter is celebrated at a different time in different countries then they must use a different calculation not a different calendar.
It is defined as the 21st of March in the calculations used to set the date of Easter - that's what I meant when I said 'not the actual day and night which are in fact equal' ie not the actual equinox (which as you say can be between 19th and 21st) If they used the actual equinox and the actual full moon Easter would be on the same day in both churches but on a different date But they don't, so it is often (not always) at a different time.
I'm in Moldova and it isn't Easter here until May.
As I think you worked out, most Romanian Orthodox people are Romanian Orthodox. Here in Moldova some are Romanian and some are Russian.
Just to add to the confusion the protestant church here celebrates easter at the same time as the Orthodox (for practical reasons I think) although they celebrate Christmas in December. Whereas in Romania most people celebrate Christmas in December and don't bother so much with January.