Easter Sunday is falling on April 21st this year, which is not the latest it can possibly be, but it is pretty close - as anybody preparing for exams and using Easter as a deadline for getting serious about study will tell you. There will be less than five full weeks of school in the last term this year which contrasts strongly with 2018 when students (and teachers) had nine weeks after returning from the Easter holiday to get themselves ready for the summer.
Which makes it all the more remarkable that the date is wrong. As I've just discovered.
Why does Easter move so much? It's all based on astronomy, of course. The bible links the events of Easter week with the Jewish feast of Passover, whose date is set every year by the first full moon after the vernal (spring) equinox. Following this tradition, Easter is fixed by most churches as the first Sunday after the first full moon after the equinox. If we take the equinox to be March 21st, this means that it can occur on any date between March 22nd and April 25th.
But there's a big 'if' in there. The vernal equinox often occurs on the 21st of March, but this varies over time due to the precession of the earth's spin, and in fact this year it fell on March 20th. And as there was a full moon on Thursday, March 21st, Easter would - if astronomical logic was being applied - have fallen on March 24th - a full week earlier than last year. If this had happened, we'd all have been back in school 3 weeks ago by now!
So why didn't it happen? Because the date of the vernal equinox was set - for the purposes of the Church - as March 21st by the Council of Nicaea , in the year 325 AD, and thats the date that we have used ever since - regardless of any wobbles in the the spin of the earth.
Links:
https://www.timeanddate.com/calendar/determining-easter-date.html
https://www.britannica.com/science/precession-of-the-equinoxes
https://publicholidays.ie/easter/
https://www.education.ie/en/Circulars-and-Forms/Active-Circulars/cl0009_2017.pdf