Each year, seven of our important feast-days have a fixed date:
Christmas Day, Epiphany, Saint Patrick's Day, The Assumption, All Saints and The Immaculate Conception.
The dates of all the other major feasts are decided by the date of Easter Sunday. Up until the year 600, Easter was celebrated on different dates throughout the world. (The date in Ireland, for instance, was not the same as the one in Rome).
So it was decided then that Easter everywhere should be fixed as
'The first Sunday after the first full moon after the 21st of
March'.
With the date of Easter known, Ash Wednesday's date is six weeks beforehand and after Easter the dates of the Ascension, Pentecost, The Most Holy Trinity and Corpus Christi fall into place.
It's only but right that Good Friday, Holy Saturday and Easter Sunday (Christ crucified, buried, risen) should form the centre of the Church's year.